diff options
Diffstat (limited to 'doc')
| -rw-r--r-- | doc/uImage.FIT/beaglebone_vboot.txt | 607 | ||||
| -rw-r--r-- | doc/uImage.FIT/signature.txt | 707 | ||||
| -rw-r--r-- | doc/usage/cmd/source.rst | 2 | ||||
| -rw-r--r-- | doc/usage/fit/beaglebone_vboot.rst | 612 | ||||
| -rw-r--r-- | doc/usage/fit/index.rst | 3 | ||||
| -rw-r--r-- | doc/usage/fit/signature.rst | 760 | ||||
| -rw-r--r-- | doc/usage/fit/verified-boot.rst (renamed from doc/uImage.FIT/verified-boot.txt) | 83 |
7 files changed, 1419 insertions, 1355 deletions
diff --git a/doc/uImage.FIT/beaglebone_vboot.txt b/doc/uImage.FIT/beaglebone_vboot.txt deleted file mode 100644 index ebd2068ed38..00000000000 --- a/doc/uImage.FIT/beaglebone_vboot.txt +++ /dev/null @@ -1,607 +0,0 @@ -Verified Boot on the Beaglebone Black -===================================== - -Introduction ------------- - -Before reading this, please read verified-boot.txt and signature.txt. These -instructions are for mainline U-Boot from v2014.07 onwards. - -There is quite a bit of documentation in this directory describing how -verified boot works in U-Boot. There is also a test which runs through the -entire process of signing an image and running U-Boot (sandbox) to check it. -However, it might be useful to also have an example on a real board. - -Beaglebone Black is a fairly common board so seems to be a reasonable choice -for an example of how to enable verified boot using U-Boot. - -First a note that may to help avoid confusion. U-Boot and Linux both use -device tree. They may use the same device tree source, but it is seldom useful -for them to use the exact same binary from the same place. More typically, -U-Boot has its device tree packaged wtih it, and the kernel's device tree is -packaged with the kernel. In particular this is important with verified boot, -since U-Boot's device tree must be immutable. If it can be changed then the -public keys can be changed and verified boot is useless. An attacker can -simply generate a new key and put his public key into U-Boot so that -everything verifies. On the other hand the kernel's device tree typically -changes when the kernel changes, so it is useful to package an updated device -tree with the kernel binary. U-Boot supports the latter with its flexible FIT -format (Flat Image Tree). - - -Overview --------- - -The steps are roughly as follows: - -1. Build U-Boot for the board, with the verified boot options enabled. - -2. Obtain a suitable Linux kernel - -3. Create a Image Tree Source file (ITS) file describing how you want the -kernel to be packaged, compressed and signed. - -4. Create a key pair - -5. Sign the kernel - -6. Put the public key into U-Boot's image - -7. Put U-Boot and the kernel onto the board - -8. Try it - - -Step 1: Build U-Boot --------------------- - -a. Set up the environment variable to point to your toolchain. You will need -this for U-Boot and also for the kernel if you build it. For example if you -installed a Linaro version manually it might be something like: - - export CROSS_COMPILE=/opt/linaro/gcc-linaro-arm-linux-gnueabihf-4.8-2013.08_linux/bin/arm-linux-gnueabihf- - -or if you just installed gcc-arm-linux-gnueabi then it might be - - export CROSS_COMPILE=arm-linux-gnueabi- - -b. Configure and build U-Boot with verified boot enabled: - - export UBOOT=/path/to/u-boot - cd $UBOOT - # You can add -j10 if you have 10 CPUs to make it faster - make O=b/am335x_boneblack_vboot am335x_boneblack_vboot_config all - export UOUT=$UBOOT/b/am335x_boneblack_vboot - -c. You will now have a U-Boot image: - - file b/am335x_boneblack_vboot/u-boot-dtb.img -b/am335x_boneblack_vboot/u-boot-dtb.img: u-boot legacy uImage, U-Boot 2014.07-rc2-00065-g2f69f8, Firmware/ARM, Firmware Image (Not compressed), 395375 bytes, Sat May 31 16:19:04 2014, Load Address: 0x80800000, Entry Point: 0x00000000, Header CRC: 0x0ABD6ACA, Data CRC: 0x36DEF7E4 - - -Step 2: Build Linux --------------------- - -a. Find the kernel image ('Image') and device tree (.dtb) file you plan to -use. In our case it is am335x-boneblack.dtb and it is built with the kernel. -At the time of writing an SD Boot image can be obtained from here: - - http://www.elinux.org/Beagleboard:Updating_The_Software#Image_For_Booting_From_microSD - -You can write this to an SD card and then mount it to extract the kernel and -device tree files. - -You can also build a kernel. Instructions for this are are here: - - http://elinux.org/Building_BBB_Kernel - -or you can use your favourite search engine. Following these instructions -produces a kernel Image and device tree files. For the record the steps were: - - export KERNEL=/path/to/kernel - cd $KERNEL - git clone git://github.com/beagleboard/kernel.git . - git checkout v3.14 - ./patch.sh - cp configs/beaglebone kernel/arch/arm/configs/beaglebone_defconfig - cd kernel - make beaglebone_defconfig - make uImage dtbs # -j10 if you have 10 CPUs - export OKERNEL=$KERNEL/kernel/arch/arm/boot - -c. You now have the 'Image' and 'am335x-boneblack.dtb' files needed to boot. - - -Step 3: Create the ITS ----------------------- - -Set up a directory for your work. - - export WORK=/path/to/dir - cd $WORK - -Put this into a file in that directory called sign.its: - -/dts-v1/; - -/ { - description = "Beaglebone black"; - #address-cells = <1>; - - images { - kernel { - data = /incbin/("Image.lzo"); - type = "kernel"; - arch = "arm"; - os = "linux"; - compression = "lzo"; - load = <0x80008000>; - entry = <0x80008000>; - hash-1 { - algo = "sha1"; - }; - }; - fdt-1 { - description = "beaglebone-black"; - data = /incbin/("am335x-boneblack.dtb"); - type = "flat_dt"; - arch = "arm"; - compression = "none"; - hash-1 { - algo = "sha1"; - }; - }; - }; - configurations { - default = "conf-1"; - conf-1 { - kernel = "kernel"; - fdt = "fdt-1"; - signature-1 { - algo = "sha1,rsa2048"; - key-name-hint = "dev"; - sign-images = "fdt", "kernel"; - }; - }; - }; -}; - - -The explanation for this is all in the documentation you have already read. -But briefly it packages a kernel and device tree, and provides a single -configuration to be signed with a key named 'dev'. The kernel is compressed -with LZO to make it smaller. - - -Step 4: Create a key pair -------------------------- - -See signature.txt for details on this step. - - cd $WORK - mkdir keys - openssl genrsa -F4 -out keys/dev.key 2048 - openssl req -batch -new -x509 -key keys/dev.key -out keys/dev.crt - -Note: keys/dev.key contains your private key and is very secret. If anyone -gets access to that file they can sign kernels with it. Keep it secure. - - -Step 5: Sign the kernel ------------------------ - -We need to use mkimage (which was built when you built U-Boot) to package the -Linux kernel into a FIT (Flat Image Tree, a flexible file format that U-Boot -can load) using the ITS file you just created. - -At the same time we must put the public key into U-Boot device tree, with the -'required' property, which tells U-Boot that this key must be verified for the -image to be valid. You will make this key available to U-Boot for booting in -step 6. - - ln -s $OKERNEL/dts/am335x-boneblack.dtb - ln -s $OKERNEL/Image - ln -s $UOUT/u-boot-dtb.img - cp $UOUT/arch/arm/dts/am335x-boneblack.dtb am335x-boneblack-pubkey.dtb - lzop Image - $UOUT/tools/mkimage -f sign.its -K am335x-boneblack-pubkey.dtb -k keys -r image.fit - -You should see something like this: - -FIT description: Beaglebone black -Created: Sun Jun 1 12:50:30 2014 - Image 0 (kernel) - Description: unavailable - Created: Sun Jun 1 12:50:30 2014 - Type: Kernel Image - Compression: lzo compressed - Data Size: 7790938 Bytes = 7608.34 kB = 7.43 MB - Architecture: ARM - OS: Linux - Load Address: 0x80008000 - Entry Point: 0x80008000 - Hash algo: sha1 - Hash value: c94364646427e10f423837e559898ef02c97b988 - Image 1 (fdt-1) - Description: beaglebone-black - Created: Sun Jun 1 12:50:30 2014 - Type: Flat Device Tree - Compression: uncompressed - Data Size: 31547 Bytes = 30.81 kB = 0.03 MB - Architecture: ARM - Hash algo: sha1 - Hash value: cb09202f889d824f23b8e4404b781be5ad38a68d - Default Configuration: 'conf-1' - Configuration 0 (conf-1) - Description: unavailable - Kernel: kernel - FDT: fdt-1 - - -Now am335x-boneblack-pubkey.dtb contains the public key and image.fit contains -the signed kernel. Jump to step 6 if you like, or continue reading to increase -your understanding. - -You can also run fit_check_sign to check it: - - $UOUT/tools/fit_check_sign -f image.fit -k am335x-boneblack-pubkey.dtb - -which results in: - -Verifying Hash Integrity ... sha1,rsa2048:dev+ -## Loading kernel from FIT Image at 7fc6ee469000 ... - Using 'conf-1' configuration - Verifying Hash Integrity ... -sha1,rsa2048:dev+ -OK - - Trying 'kernel' kernel subimage - Description: unavailable - Created: Sun Jun 1 12:50:30 2014 - Type: Kernel Image - Compression: lzo compressed - Data Size: 7790938 Bytes = 7608.34 kB = 7.43 MB - Architecture: ARM - OS: Linux - Load Address: 0x80008000 - Entry Point: 0x80008000 - Hash algo: sha1 - Hash value: c94364646427e10f423837e559898ef02c97b988 - Verifying Hash Integrity ... -sha1+ -OK - -Unimplemented compression type 4 -## Loading fdt from FIT Image at 7fc6ee469000 ... - Using 'conf-1' configuration - Trying 'fdt-1' fdt subimage - Description: beaglebone-black - Created: Sun Jun 1 12:50:30 2014 - Type: Flat Device Tree - Compression: uncompressed - Data Size: 31547 Bytes = 30.81 kB = 0.03 MB - Architecture: ARM - Hash algo: sha1 - Hash value: cb09202f889d824f23b8e4404b781be5ad38a68d - Verifying Hash Integrity ... -sha1+ -OK - - Loading Flat Device Tree ... OK - -## Loading ramdisk from FIT Image at 7fc6ee469000 ... - Using 'conf-1' configuration -Could not find subimage node - -Signature check OK - - -At the top, you see "sha1,rsa2048:dev+". This means that it checked an RSA key -of size 2048 bits using SHA1 as the hash algorithm. The key name checked was -'dev' and the '+' means that it verified. If it showed '-' that would be bad. - -Once the configuration is verified it is then possible to rely on the hashes -in each image referenced by that configuration. So fit_check_sign goes on to -load each of the images. We have a kernel and an FDT but no ramkdisk. In each -case fit_check_sign checks the hash and prints sha1+ meaning that the SHA1 -hash verified. This means that none of the images has been tampered with. - -There is a test in test/vboot which uses U-Boot's sandbox build to verify that -the above flow works. - -But it is fun to do this by hand, so you can load image.fit into a hex editor -like ghex, and change a byte in the kernel: - - $UOUT/tools/fit_info -f image.fit -n /images/kernel -p data -NAME: kernel -LEN: 7790938 -OFF: 168 - -This tells us that the kernel starts at byte offset 168 (decimal) in image.fit -and extends for about 7MB. Try changing a byte at 0x2000 (say) and run -fit_check_sign again. You should see something like: - -Verifying Hash Integrity ... sha1,rsa2048:dev+ -## Loading kernel from FIT Image at 7f5a39571000 ... - Using 'conf-1' configuration - Verifying Hash Integrity ... -sha1,rsa2048:dev+ -OK - - Trying 'kernel' kernel subimage - Description: unavailable - Created: Sun Jun 1 13:09:21 2014 - Type: Kernel Image - Compression: lzo compressed - Data Size: 7790938 Bytes = 7608.34 kB = 7.43 MB - Architecture: ARM - OS: Linux - Load Address: 0x80008000 - Entry Point: 0x80008000 - Hash algo: sha1 - Hash value: c94364646427e10f423837e559898ef02c97b988 - Verifying Hash Integrity ... -sha1 error -Bad hash value for 'hash-1' hash node in 'kernel' image node -Bad Data Hash - -## Loading fdt from FIT Image at 7f5a39571000 ... - Using 'conf-1' configuration - Trying 'fdt-1' fdt subimage - Description: beaglebone-black - Created: Sun Jun 1 13:09:21 2014 - Type: Flat Device Tree - Compression: uncompressed - Data Size: 31547 Bytes = 30.81 kB = 0.03 MB - Architecture: ARM - Hash algo: sha1 - Hash value: cb09202f889d824f23b8e4404b781be5ad38a68d - Verifying Hash Integrity ... -sha1+ -OK - - Loading Flat Device Tree ... OK - -## Loading ramdisk from FIT Image at 7f5a39571000 ... - Using 'conf-1' configuration -Could not find subimage node - -Signature check Bad (error 1) - - -It has detected the change in the kernel. - -You can also be sneaky and try to switch images, using the libfdt utilities -that come with dtc (package name is device-tree-compiler but you will need a -recent version like 1.4: - - dtc -v -Version: DTC 1.4.0 - -First we can check which nodes are actually hashed by the configuration: - - fdtget -l image.fit / -images -configurations - - fdtget -l image.fit /configurations -conf-1 -fdtget -l image.fit /configurations/conf-1 -signature-1 - - fdtget -p image.fit /configurations/conf-1/signature-1 -hashed-strings -hashed-nodes -timestamp -signer-version -signer-name -value -algo -key-name-hint -sign-images - - fdtget image.fit /configurations/conf-1/signature-1 hashed-nodes -/ /configurations/conf-1 /images/fdt-1 /images/fdt-1/hash /images/kernel /images/kernel/hash-1 - -This gives us a bit of a look into the signature that mkimage added. Note you -can also use fdtdump to list the entire device tree. - -Say we want to change the kernel that this configuration uses -(/images/kernel). We could just put a new kernel in the image, but we will -need to change the hash to match. Let's simulate that by changing a byte of -the hash: - - fdtget -tx image.fit /images/kernel/hash-1 value -c9436464 6427e10f 423837e5 59898ef0 2c97b988 - fdtput -tx image.fit /images/kernel/hash-1 value c9436464 6427e10f 423837e5 59898ef0 2c97b981 - -Now check it again: - - $UOUT/tools/fit_check_sign -f image.fit -k am335x-boneblack-pubkey.dtb -Verifying Hash Integrity ... sha1,rsa2048:devrsa_verify_with_keynode: RSA failed to verify: -13 -rsa_verify_with_keynode: RSA failed to verify: -13 -- -Failed to verify required signature 'key-dev' -Signature check Bad (error 1) - -This time we don't even get as far as checking the images, since the -configuration signature doesn't match. We can't change any hashes without the -signature check noticing. The configuration is essentially locked. U-Boot has -a public key for which it requires a match, and will not permit the use of any -configuration that does not match that public key. The only way the -configuration will match is if it was signed by the matching private key. - -It would also be possible to add a new signature node that does match your new -configuration. But that won't work since you are not allowed to change the -configuration in any way. Try it with a fresh (valid) image if you like by -running the mkimage link again. Then: - - fdtput -p image.fit /configurations/conf-1/signature-1 value fred - $UOUT/tools/fit_check_sign -f image.fit -k am335x-boneblack-pubkey.dtb -Verifying Hash Integrity ... - -sha1,rsa2048:devrsa_verify_with_keynode: RSA failed to verify: -13 -rsa_verify_with_keynode: RSA failed to verify: -13 -- -Failed to verify required signature 'key-dev' -Signature check Bad (error 1) - - -Of course it would be possible to add an entirely new configuration and boot -with that, but it still needs to be signed, so it won't help. - - -6. Put the public key into U-Boot's image ------------------------------------------ - -Having confirmed that the signature is doing its job, let's try it out in -U-Boot on the board. U-Boot needs access to the public key corresponding to -the private key that you signed with so that it can verify any kernels that -you sign. - - cd $UBOOT - make O=b/am335x_boneblack_vboot EXT_DTB=${WORK}/am335x-boneblack-pubkey.dtb - -Here we are overriding the normal device tree file with our one, which -contains the public key. - -Now you have a special U-Boot image with the public key. It can verify can -kernel that you sign with the private key as in step 5. - -If you like you can take a look at the public key information that mkimage -added to U-Boot's device tree: - - fdtget -p am335x-boneblack-pubkey.dtb /signature/key-dev -required -algo -rsa,r-squared -rsa,modulus -rsa,n0-inverse -rsa,num-bits -key-name-hint - -This has information about the key and some pre-processed values which U-Boot -can use to verify against it. These values are obtained from the public key -certificate by mkimage, but require quite a bit of code to generate. To save -code space in U-Boot, the information is extracted and written in raw form for -U-Boot to easily use. The same mechanism is used in Google's Chrome OS. - -Notice the 'required' property. This marks the key as required - U-Boot will -not boot any image that does not verify against this key. - - -7. Put U-Boot and the kernel onto the board -------------------------------------------- - -The method here varies depending on how you are booting. For this example we -are booting from an micro-SD card with two partitions, one for U-Boot and one -for Linux. Put it into your machine and write U-Boot and the kernel to it. -Here the card is /dev/sde: - - cd $WORK - export UDEV=/dev/sde1 # Change thes two lines to the correct device - export KDEV=/dev/sde2 - sudo mount $UDEV /mnt/tmp && sudo cp $UOUT/u-boot-dtb.img /mnt/tmp/u-boot.img && sleep 1 && sudo umount $UDEV - sudo mount $KDEV /mnt/tmp && sudo cp $WORK/image.fit /mnt/tmp/boot/image.fit && sleep 1 && sudo umount $KDEV - - -8. Try it ---------- - -Boot the board using the commands below: - - setenv bootargs console=ttyO0,115200n8 quiet root=/dev/mmcblk0p2 ro rootfstype=ext4 rootwait - ext2load mmc 0:2 82000000 /boot/image.fit - bootm 82000000 - -You should then see something like this: - -U-Boot# setenv bootargs console=ttyO0,115200n8 quiet root=/dev/mmcblk0p2 ro rootfstype=ext4 rootwait -U-Boot# ext2load mmc 0:2 82000000 /boot/image.fit -7824930 bytes read in 589 ms (12.7 MiB/s) -U-Boot# bootm 82000000 -## Loading kernel from FIT Image at 82000000 ... - Using 'conf-1' configuration - Verifying Hash Integrity ... sha1,rsa2048:dev+ OK - Trying 'kernel' kernel subimage - Description: unavailable - Created: 2014-06-01 19:32:54 UTC - Type: Kernel Image - Compression: lzo compressed - Data Start: 0x820000a8 - Data Size: 7790938 Bytes = 7.4 MiB - Architecture: ARM - OS: Linux - Load Address: 0x80008000 - Entry Point: 0x80008000 - Hash algo: sha1 - Hash value: c94364646427e10f423837e559898ef02c97b988 - Verifying Hash Integrity ... sha1+ OK -## Loading fdt from FIT Image at 82000000 ... - Using 'conf-1' configuration - Trying 'fdt-1' fdt subimage - Description: beaglebone-black - Created: 2014-06-01 19:32:54 UTC - Type: Flat Device Tree - Compression: uncompressed - Data Start: 0x8276e2ec - Data Size: 31547 Bytes = 30.8 KiB - Architecture: ARM - Hash algo: sha1 - Hash value: cb09202f889d824f23b8e4404b781be5ad38a68d - Verifying Hash Integrity ... sha1+ OK - Booting using the fdt blob at 0x8276e2ec - Uncompressing Kernel Image ... OK - Loading Device Tree to 8fff5000, end 8ffffb3a ... OK - -Starting kernel ... - -[ 0.582377] omap_init_mbox: hwmod doesn't have valid attrs -[ 2.589651] musb-hdrc musb-hdrc.0.auto: Failed to request rx1. -[ 2.595830] musb-hdrc musb-hdrc.0.auto: musb_init_controller failed with status -517 -[ 2.606470] musb-hdrc musb-hdrc.1.auto: Failed to request rx1. -[ 2.612723] musb-hdrc musb-hdrc.1.auto: musb_init_controller failed with status -517 -[ 2.940808] drivers/rtc/hctosys.c: unable to open rtc device (rtc0) -[ 7.248889] libphy: PHY 4a101000.mdio:01 not found -[ 7.253995] net eth0: phy 4a101000.mdio:01 not found on slave 1 -systemd-fsck[83]: Angstrom: clean, 50607/218160 files, 306348/872448 blocks - -.---O---. -| | .-. o o -| | |-----.-----.-----.| | .----..-----.-----. -| | | __ | ---'| '--.| .-'| | | -| | | | | |--- || --'| | | ' | | | | -'---'---'--'--'--. |-----''----''--' '-----'-'-'-' - -' | - '---' - -The Angstrom Distribution beaglebone ttyO0 - -Angstrom v2012.12 - Kernel 3.14.1+ - -beaglebone login: - -At this point your kernel has been verified and you can be sure that it is one -that you signed. As an exercise, try changing image.fit as in step 5 and see -what happens. - - -Further Improvements --------------------- - -Several of the steps here can be easily automated. In particular it would be -capital if signing and packaging a kernel were easy, perhaps a simple make -target in the kernel. - -Some mention of how to use multiple .dtb files in a FIT might be useful. - -U-Boot's verified boot mechanism has not had a robust and independent security -review. Such a review should look at the implementation and its resistance to -attacks. - -Perhaps the verified boot feature could could be integrated into the Amstrom -distribution. - - -Simon Glass -sjg@chromium.org -2-June-14 diff --git a/doc/uImage.FIT/signature.txt b/doc/uImage.FIT/signature.txt deleted file mode 100644 index 21eb3894aad..00000000000 --- a/doc/uImage.FIT/signature.txt +++ /dev/null @@ -1,707 +0,0 @@ -U-Boot FIT Signature Verification -================================= - -Introduction ------------- -FIT supports hashing of images so that these hashes can be checked on -loading. This protects against corruption of the image. However it does not -prevent the substitution of one image for another. - -The signature feature allows the hash to be signed with a private key such -that it can be verified using a public key later. Provided that the private -key is kept secret and the public key is stored in a non-volatile place, -any image can be verified in this way. - -See verified-boot.txt for more general information on verified boot. - - -Concepts --------- -Some familiarity with public key cryptography is assumed in this section. - -The procedure for signing is as follows: - - - hash an image in the FIT - - sign the hash with a private key to produce a signature - - store the resulting signature in the FIT - -The procedure for verification is: - - - read the FIT - - obtain the public key - - extract the signature from the FIT - - hash the image from the FIT - - verify (with the public key) that the extracted signature matches the - hash - -The signing is generally performed by mkimage, as part of making a firmware -image for the device. The verification is normally done in U-Boot on the -device. - - -Algorithms ----------- -In principle any suitable algorithm can be used to sign and verify a hash. -U-Boot supports a few hashing and verification algorithms. See below for -details. - -While it is acceptable to bring in large cryptographic libraries such as -openssl on the host side (e.g. mkimage), it is not desirable for U-Boot. -For the run-time verification side, it is important to keep code and data -size as small as possible. - -For this reason the RSA image verification uses pre-processed public keys -which can be used with a very small amount of code - just some extraction -of data from the FDT and exponentiation mod n. Code size impact is a little -under 5KB on Tegra Seaboard, for example. - -It is relatively straightforward to add new algorithms if required. If -another RSA variant is needed, then it can be added with the -U_BOOT_CRYPTO_ALGO() macro. If another algorithm is needed (such as DSA) then -it can be placed in a directory alongside lib/rsa/, and its functions added -using U_BOOT_CRYPTO_ALGO(). - - -Creating an RSA key pair and certificate ----------------------------------------- -To create a new public/private key pair, size 2048 bits: - -$ openssl genpkey -algorithm RSA -out keys/dev.key \ - -pkeyopt rsa_keygen_bits:2048 -pkeyopt rsa_keygen_pubexp:65537 - -To create a certificate for this containing the public key: - -$ openssl req -batch -new -x509 -key keys/dev.key -out keys/dev.crt - -If you like you can look at the public key also: - -$ openssl rsa -in keys/dev.key -pubout - - -Device Tree Bindings --------------------- -The following properties are required in the FIT's signature node(s) to -allow the signer to operate. These should be added to the .its file. -Signature nodes sit at the same level as hash nodes and are called -signature-1, signature-2, etc. - -- algo: Algorithm name (e.g. "sha1,rsa2048") - -- key-name-hint: Name of key to use for signing. The keys will normally be in -a single directory (parameter -k to mkimage). For a given key <name>, its -private key is stored in <name>.key and the certificate is stored in -<name>.crt. - -When the image is signed, the following properties are added (mandatory): - -- value: The signature data (e.g. 256 bytes for 2048-bit RSA) - -When the image is signed, the following properties are optional: - -- timestamp: Time when image was signed (standard Unix time_t format) - -- signer-name: Name of the signer (e.g. "mkimage") - -- signer-version: Version string of the signer (e.g. "2013.01") - -- comment: Additional information about the signer or image - -- padding: The padding algorithm, it may be pkcs-1.5 or pss, - if no value is provided we assume pkcs-1.5 - -For config bindings (see Signed Configurations below), the following -additional properties are optional: - -- sign-images: A list of images to sign, each being a property of the conf -node that contains then. The default is "kernel,fdt" which means that these -two images will be looked up in the config and signed if present. - -For config bindings, these properties are added by the signer: - -- hashed-nodes: A list of nodes which were hashed by the signer. Each is - a string - the full path to node. A typical value might be: - - hashed-nodes = "/", "/configurations/conf-1", "/images/kernel", - "/images/kernel/hash-1", "/images/fdt-1", - "/images/fdt-1/hash-1"; - -- hashed-strings: The start and size of the string region of the FIT that - was hashed - -Example: See sign-images.its for an example image tree source file and -sign-configs.its for config signing. - - -Public Key Storage ------------------- -In order to verify an image that has been signed with a public key we need to -have a trusted public key. This cannot be stored in the signed image, since -it would be easy to alter. For this implementation we choose to store the -public key in U-Boot's control FDT (using CONFIG_OF_CONTROL). - -Public keys should be stored as sub-nodes in a /signature node. Required -properties are: - -- algo: Algorithm name (e.g. "sha1,rsa2048" or "sha256,ecdsa256") - -Optional properties are: - -- key-name-hint: Name of key used for signing. This is only a hint since it -is possible for the name to be changed. Verification can proceed by checking -all available signing keys until one matches. - -- required: If present this indicates that the key must be verified for the -image / configuration to be considered valid. Only required keys are -normally verified by the FIT image booting algorithm. Valid values are -"image" to force verification of all images, and "conf" to force verification -of the selected configuration (which then relies on hashes in the images to -verify those). - -Each signing algorithm has its own additional properties. - -For RSA the following are mandatory: - -- rsa,num-bits: Number of key bits (e.g. 2048) -- rsa,modulus: Modulus (N) as a big-endian multi-word integer -- rsa,exponent: Public exponent (E) as a 64 bit unsigned integer -- rsa,r-squared: (2^num-bits)^2 as a big-endian multi-word integer -- rsa,n0-inverse: -1 / modulus[0] mod 2^32 - -For ECDSA the following are mandatory: -- ecdsa,curve: Name of ECDSA curve (e.g. "prime256v1") -- ecdsa,x-point: Public key X coordinate as a big-endian multi-word integer -- ecdsa,y-point: Public key Y coordinate as a big-endian multi-word integer - -These parameters can be added to a binary device tree using parameter -K of the -mkimage command:: - - tools/mkimage -f fit.its -K control.dtb -k keys -r image.fit - -Here is an example of a generated device tree node:: - - signature { - key-dev { - required = "conf"; - algo = "sha256,rsa2048"; - rsa,r-squared = <0xb76d1acf 0xa1763ca5 0xeb2f126 - 0x742edc80 0xd3f42177 0x9741d9d9 - 0x35bb476e 0xff41c718 0xd3801430 - 0xf22537cb 0xa7e79960 0xae32a043 - 0x7da1427a 0x341d6492 0x3c2762f5 - 0xaac04726 0x5b262d96 0xf984e86d - 0xb99443c7 0x17080c33 0x940f6892 - 0xd57a95d1 0x6ea7b691 0xc5038fa8 - 0x6bb48a6e 0x73f1b1ea 0x37160841 - 0xe05715ce 0xa7c45bbd 0x690d82d5 - 0x99c2454c 0x6ff117b3 0xd830683b - 0x3f81c9cf 0x1ca38a91 0x0c3392e4 - 0xd817c625 0x7b8e9a24 0x175b89ea - 0xad79f3dc 0x4d50d7b4 0x9d4e90f8 - 0xad9e2939 0xc165d6a4 0x0ada7e1b - 0xfb1bf495 0xfc3131c2 0xb8c6e604 - 0xc2761124 0xf63de4a6 0x0e9565f9 - 0xc8e53761 0x7e7a37a5 0xe99dcdae - 0x9aff7e1e 0xbd44b13d 0x6b0e6aa4 - 0x038907e4 0x8e0d6850 0xef51bc20 - 0xf73c94af 0x88bea7b1 0xcbbb1b30 - 0xd024b7f3>; - rsa,modulus = <0xc0711d6cb 0x9e86db7f 0x45986dbe - 0x023f1e8c9 0xe1a4c4d0 0x8a0dfdc9 - 0x023ba0c48 0x06815f6a 0x5caa0654 - 0x07078c4b7 0x3d154853 0x40729023 - 0x0b007c8fe 0x5a3647e5 0x23b41e20 - 0x024720591 0x66915305 0x0e0b29b0 - 0x0de2ad30d 0x8589430f 0xb1590325 - 0x0fb9f5d5e 0x9eba752a 0xd88e6de9 - 0x056b3dcc6 0x9a6b8e61 0x6784f61f - 0x000f39c21 0x5eec6b33 0xd78e4f78 - 0x0921a305f 0xaa2cc27e 0x1ca917af - 0x06e1134f4 0xd48cac77 0x4e914d07 - 0x0f707aa5a 0x0d141f41 0x84677f1d - 0x0ad47a049 0x028aedb6 0xd5536fcf - 0x03fef1e4f 0x133a03d2 0xfd7a750a - 0x0f9159732 0xd207812e 0x6a807375 - 0x06434230d 0xc8e22dad 0x9f29b3d6 - 0x07c44ac2b 0xfa2aad88 0xe2429504 - 0x041febd41 0x85d0d142 0x7b194d65 - 0x06e5d55ea 0x41116961 0xf3181dde - 0x068bf5fbc 0x3dd82047 0x00ee647e - 0x0d7a44ab3>; - rsa,exponent = <0x00 0x10001>; - rsa,n0-inverse = <0xb3928b85>; - rsa,num-bits = <0x800>; - key-name-hint = "dev"; - }; - }; - - -Signed Configurations ---------------------- -While signing images is useful, it does not provide complete protection -against several types of attack. For example, it it possible to create a -FIT with the same signed images, but with the configuration changed such -that a different one is selected (mix and match attack). It is also possible -to substitute a signed image from an older FIT version into a newer FIT -(roll-back attack). - -As an example, consider this FIT: - -/ { - images { - kernel-1 { - data = <data for kernel1> - signature-1 { - algo = "sha1,rsa2048"; - value = <...kernel signature 1...> - }; - }; - kernel-2 { - data = <data for kernel2> - signature-1 { - algo = "sha1,rsa2048"; - value = <...kernel signature 2...> - }; - }; - fdt-1 { - data = <data for fdt1>; - signature-1 { - algo = "sha1,rsa2048"; - value = <...fdt signature 1...> - }; - }; - fdt-2 { - data = <data for fdt2>; - signature-1 { - algo = "sha1,rsa2048"; - value = <...fdt signature 2...> - }; - }; - }; - configurations { - default = "conf-1"; - conf-1 { - kernel = "kernel-1"; - fdt = "fdt-1"; - }; - conf-2 { - kernel = "kernel-2"; - fdt = "fdt-2"; - }; - }; -}; - -Since both kernels are signed it is easy for an attacker to add a new -configuration 3 with kernel 1 and fdt 2: - - configurations { - default = "conf-1"; - conf-1 { - kernel = "kernel-1"; - fdt = "fdt-1"; - }; - conf-2 { - kernel = "kernel-2"; - fdt = "fdt-2"; - }; - conf-3 { - kernel = "kernel-1"; - fdt = "fdt-2"; - }; - }; - -With signed images, nothing protects against this. Whether it gains an -advantage for the attacker is debatable, but it is not secure. - -To solve this problem, we support signed configurations. In this case it -is the configurations that are signed, not the image. Each image has its -own hash, and we include the hash in the configuration signature. - -So the above example is adjusted to look like this: - -/ { - images { - kernel-1 { - data = <data for kernel1> - hash-1 { - algo = "sha1"; - value = <...kernel hash 1...> - }; - }; - kernel-2 { - data = <data for kernel2> - hash-1 { - algo = "sha1"; - value = <...kernel hash 2...> - }; - }; - fdt-1 { - data = <data for fdt1>; - hash-1 { - algo = "sha1"; - value = <...fdt hash 1...> - }; - }; - fdt-2 { - data = <data for fdt2>; - hash-1 { - algo = "sha1"; - value = <...fdt hash 2...> - }; - }; - }; - configurations { - default = "conf-1"; - conf-1 { - kernel = "kernel-1"; - fdt = "fdt-1"; - signature-1 { - algo = "sha1,rsa2048"; - value = <...conf 1 signature...>; - }; - }; - conf-2 { - kernel = "kernel-2"; - fdt = "fdt-2"; - signature-1 { - algo = "sha1,rsa2048"; - value = <...conf 1 signature...>; - }; - }; - }; -}; - - -You can see that we have added hashes for all images (since they are no -longer signed), and a signature to each configuration. In the above example, -mkimage will sign configurations/conf-1, the kernel and fdt that are -pointed to by the configuration (/images/kernel-1, /images/kernel-1/hash-1, -/images/fdt-1, /images/fdt-1/hash-1) and the root structure of the image -(so that it isn't possible to add or remove root nodes). The signature is -written into /configurations/conf-1/signature-1/value. It can easily be -verified later even if the FIT has been signed with other keys in the -meantime. - - -Details -------- -The signature node contains a property ('hashed-nodes') which lists all the -nodes that the signature was made over. The image is walked in order and each -tag processed as follows: -- DTB_BEGIN_NODE: The tag and the following name are included in the signature - if the node or its parent are present in 'hashed-nodes' -- DTB_END_NODE: The tag is included in the signature if the node or its parent - are present in 'hashed-nodes' -- DTB_PROPERTY: The tag, the length word, the offset in the string table, and - the data are all included if the current node is present in 'hashed-nodes' - and the property name is not 'data'. -- DTB_END: The tag is always included in the signature. -- DTB_NOP: The tag is included in the signature if the current node is present - in 'hashed-nodes' - -In addition, the signature contains a property 'hashed-strings' which contains -the offset and length in the string table of the strings that are to be -included in the signature (this is done last). - -IMPORTANT: To verify the signature outside u-boot, it is vital to not only -calculate the hash of the image and verify the signature with that, but also to -calculate the hashes of the kernel, fdt, and ramdisk images and check those -match the hash values in the corresponding 'hash*' subnodes. - - -Verification ------------- -FITs are verified when loaded. After the configuration is selected a list -of required images is produced. If there are 'required' public keys, then -each image must be verified against those keys. This means that every image -that might be used by the target needs to be signed with 'required' keys. - -This happens automatically as part of a bootm command when FITs are used. - -For Signed Configurations, the default verification behavior can be changed by -the following optional property in /signature node in U-Boot's control FDT. - -- required-mode: Valid values are "any" to allow verified boot to succeed if -the selected configuration is signed by any of the 'required' keys, and "all" -to allow verified boot to succeed if the selected configuration is signed by -all of the 'required' keys. - -This property can be added to a binary device tree using fdtput as shown in -below examples:: - - fdtput -t s control.dtb /signature required-mode any - fdtput -t s control.dtb /signature required-mode all - - -Enabling FIT Verification -------------------------- -In addition to the options to enable FIT itself, the following CONFIGs must -be enabled: - -CONFIG_FIT_SIGNATURE - enable signing and verification in FITs -CONFIG_RSA - enable RSA algorithm for signing -CONFIG_ECDSA - enable ECDSA algorithm for signing - -WARNING: When relying on signed FIT images with required signature check -the legacy image format is default disabled by not defining -CONFIG_LEGACY_IMAGE_FORMAT - - -Testing -------- -An easy way to test signing and verification is to use the test script -provided in test/vboot/vboot_test.sh. This uses sandbox (a special version -of U-Boot which runs under Linux) to show the operation of a 'bootm' -command loading and verifying images. - -A sample run is show below: - -$ make O=sandbox sandbox_config -$ make O=sandbox -$ O=sandbox ./test/vboot/vboot_test.sh - - -Simple Verified Boot Test -========================= - -Please see doc/uImage.FIT/verified-boot.txt for more information - -/home/hs/ids/u-boot/sandbox/tools/mkimage -D -I dts -O dtb -p 2000 -Build keys -do sha1 test -Build FIT with signed images -Test Verified Boot Run: unsigned signatures:: OK -Sign images -Test Verified Boot Run: signed images: OK -Build FIT with signed configuration -Test Verified Boot Run: unsigned config: OK -Sign images -Test Verified Boot Run: signed config: OK -check signed config on the host -Signature check OK -OK -Test Verified Boot Run: signed config: OK -Test Verified Boot Run: signed config with bad hash: OK -do sha256 test -Build FIT with signed images -Test Verified Boot Run: unsigned signatures:: OK -Sign images -Test Verified Boot Run: signed images: OK -Build FIT with signed configuration -Test Verified Boot Run: unsigned config: OK -Sign images -Test Verified Boot Run: signed config: OK -check signed config on the host -Signature check OK -OK -Test Verified Boot Run: signed config: OK -Test Verified Boot Run: signed config with bad hash: OK - -Test passed - - -Software signing: keydir vs keyfile ------------------------------------ - -In the simplest case, signing is done by giving mkimage the 'keyfile'. This is -the path to a file containing the signing key. - -The alternative is to pass the 'keydir' argument. In this case the filename of -the key is derived from the 'keydir' and the "key-name-hint" property in the -FIT. In this case the "key-name-hint" property is mandatory, and the key must -exist in "<keydir>/<key-name-hint>.<ext>" Here the extension "ext" is -specific to the signing algorithm. - - -Hardware Signing with PKCS#11 or with HSM ------------------------------------------ - -Securely managing private signing keys can challenging, especially when the -keys are stored on the file system of a computer that is connected to the -Internet. If an attacker is able to steal the key, they can sign malicious FIT -images which will appear genuine to your devices. - -An alternative solution is to keep your signing key securely stored on hardware -device like a smartcard, USB token or Hardware Security Module (HSM) and have -them perform the signing. PKCS#11 is standard for interfacing with these crypto -device. - -Requirements: -Smartcard/USB token/HSM which can work with some openssl engine -openssl - -For pkcs11 engine usage: -libp11 (provides pkcs11 engine) -p11-kit (recommended to simplify setup) -opensc (for smartcards and smartcard like USB devices) -gnutls (recommended for key generation, p11tool) - -For generic HSMs respective openssl engine must be installed and locateable by -openssl. This may require setting up LD_LIBRARY_PATH if engine is not installed -to openssl's default search paths. - -PKCS11 engine support forms "key id" based on "keydir" and with -"key-name-hint". "key-name-hint" is used as "object" name (if not defined in -keydir). "keydir" (if defined) is used to define (prefix for) which PKCS11 source -is being used for lookup up for the key. - -PKCS11 engine key ids: - "pkcs11:<keydir>;object=<key-name-hint>;type=<public|private>" -or, if keydir contains "object=" - "pkcs11:<keydir>;type=<public|private>" -or - "pkcs11:object=<key-name-hint>;type=<public|private>", - -Generic HSM engine support forms "key id" based on "keydir" and with -"key-name-hint". If "keydir" is specified for mkimage it is used as a prefix in -"key id" and is appended with "key-name-hint". - -Generic engine key ids: - "<keydir><key-name-hint>" -or - "<key-name-hint>" - -In order to set the pin in the HSM, an environment variable "MKIMAGE_SIGN_PIN" -can be specified. - -The following examples use the Nitrokey Pro using pkcs11 engine. Instructions -for other devices may vary. - -Notes on pkcs11 engine setup: - -Make sure p11-kit, opensc are installed and that p11-kit is setup to use opensc. -/usr/share/p11-kit/modules/opensc.module should be present on your system. - - -Generating Keys On the Nitrokey: - -$ gpg --card-edit - -Reader ...........: Nitrokey Nitrokey Pro (xxxxxxxx0000000000000000) 00 00 -Application ID ...: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx -Version ..........: 2.1 -Manufacturer .....: ZeitControl -Serial number ....: xxxxxxxx -Name of cardholder: [not set] -Language prefs ...: de -Sex ..............: unspecified -URL of public key : [not set] -Login data .......: [not set] -Signature PIN ....: forced -Key attributes ...: rsa2048 rsa2048 rsa2048 -Max. PIN lengths .: 32 32 32 -PIN retry counter : 3 0 3 -Signature counter : 0 -Signature key ....: [none] -Encryption key....: [none] -Authentication key: [none] -General key info..: [none] - -gpg/card> generate -Make off-card backup of encryption key? (Y/n) n - -Please note that the factory settings of the PINs are - PIN = '123456' Admin PIN = '12345678' -You should change them using the command --change-pin - -What keysize do you want for the Signature key? (2048) 4096 -The card will now be re-configured to generate a key of 4096 bits -Note: There is no guarantee that the card supports the requested size. - If the key generation does not succeed, please check the - documentation of your card to see what sizes are allowed. -What keysize do you want for the Encryption key? (2048) 4096 -The card will now be re-configured to generate a key of 4096 bits -What keysize do you want for the Authentication key? (2048) 4096 -The card will now be re-configured to generate a key of 4096 bits -Please specify how long the key should be valid. - 0 = key does not expire - <n> = key expires in n days - <n>w = key expires in n weeks - <n>m = key expires in n months - <n>y = key expires in n years -Key is valid for? (0) -Key does not expire at all -Is this correct? (y/N) y - -GnuPG needs to construct a user ID to identify your key. - -Real name: John Doe -Email address: john.doe@email.com -Comment: -You selected this USER-ID: - "John Doe <john.doe@email.com>" - -Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? o - - -Using p11tool to get the token URL: - -Depending on system configuration, gpg-agent may need to be killed first. - -$ p11tool --provider /usr/lib/opensc-pkcs11.so --list-tokens -Token 0: -URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29 -Label: OpenPGP card (User PIN (sig)) -Type: Hardware token -Manufacturer: ZeitControl -Model: PKCS#15 emulated -Serial: 000xxxxxxxxx -Module: (null) - - -Token 1: -URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%29 -Label: OpenPGP card (User PIN) -Type: Hardware token -Manufacturer: ZeitControl -Model: PKCS#15 emulated -Serial: 000xxxxxxxxx -Module: (null) - -Use the portion of the signature token URL after "pkcs11:" as the keydir argument (-k) to mkimage below. - - -Use the URL of the token to list the private keys: - -$ p11tool --login --provider /usr/lib/opensc-pkcs11.so --list-privkeys \ -"pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29" -Token 'OpenPGP card (User PIN (sig))' with URL 'pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29' requires user PIN -Enter PIN: -Object 0: -URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29;id=%01;object=Signature%20key;type=private -Type: Private key -Label: Signature key -Flags: CKA_PRIVATE; CKA_NEVER_EXTRACTABLE; CKA_SENSITIVE; -ID: 01 - -Use the label, in this case "Signature key" as the key-name-hint in your FIT. - -Create the fitImage: -$ ./tools/mkimage -f fit-image.its fitImage - - -Sign the fitImage with the hardware key: - -$ ./tools/mkimage -F -k \ -"model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29" \ --K u-boot.dtb -N pkcs11 -r fitImage - - -Future Work ------------ -- Roll-back protection using a TPM is done using the tpm command. This can -be scripted, but we might consider a default way of doing this, built into -bootm. - - -Possible Future Work --------------------- -- More sandbox tests for failure modes -- Passwords for keys/certificates -- Perhaps implement OAEP -- Enhance bootm to permit scripted signature verification (so that a script -can verify an image but not actually boot it) - - -Simon Glass -sjg@chromium.org -1-1-13 diff --git a/doc/usage/cmd/source.rst b/doc/usage/cmd/source.rst index 61a45059096..6f5fa285134 100644 --- a/doc/usage/cmd/source.rst +++ b/doc/usage/cmd/source.rst @@ -22,7 +22,7 @@ Two formats for script files exist: * Flat Image Tree (FIT) The benefit of the FIT images is that they can be signed and verifed as -decribed in :download:`signature.txt <../../uImage.FIT/signature.txt>`. +described in :doc:`../fit/signature`. Both formats can be created with the mkimage tool. diff --git a/doc/usage/fit/beaglebone_vboot.rst b/doc/usage/fit/beaglebone_vboot.rst new file mode 100644 index 00000000000..0580ee10bdc --- /dev/null +++ b/doc/usage/fit/beaglebone_vboot.rst @@ -0,0 +1,612 @@ +.. SPDX-License-Identifier: GPL-2.0+ + +Verified Boot on the Beaglebone Black +===================================== + +Introduction +------------ + +Before reading this, please read :doc:`verified-boot` and :doc:`signature`. +These instructions are for mainline U-Boot from v2014.07 onwards. + +There is quite a bit of documentation in this directory describing how +verified boot works in U-Boot. There is also a test which runs through the +entire process of signing an image and running U-Boot (sandbox) to check it. +However, it might be useful to also have an example on a real board. + +Beaglebone Black is a fairly common board so seems to be a reasonable choice +for an example of how to enable verified boot using U-Boot. + +First a note that may to help avoid confusion. U-Boot and Linux both use +device tree. They may use the same device tree source, but it is seldom useful +for them to use the exact same binary from the same place. More typically, +U-Boot has its device tree packaged with it, and the kernel's device tree is +packaged with the kernel. In particular this is important with verified boot, +since U-Boot's device tree must be immutable. If it can be changed then the +public keys can be changed and verified boot is useless. An attacker can +simply generate a new key and put his public key into U-Boot so that +everything verifies. On the other hand the kernel's device tree typically +changes when the kernel changes, so it is useful to package an updated device +tree with the kernel binary. U-Boot supports the latter with its flexible FIT +format (Flat Image Tree). + + +Overview +-------- + +The steps are roughly as follows: + +#. Build U-Boot for the board, with the verified boot options enabled. + +#. Obtain a suitable Linux kernel + +#. Create a Image Tree Source file (ITS) file describing how you want the + kernel to be packaged, compressed and signed. + +#. Create a key pair + +#. Sign the kernel + +#. Put the public key into U-Boot's image + +#. Put U-Boot and the kernel onto the board + +#. Try it + + +Step 1: Build U-Boot +-------------------- + +a. Set up the environment variable to point to your toolchain. You will need + this for U-Boot and also for the kernel if you build it. For example if you + installed a Linaro version manually it might be something like:: + + export CROSS_COMPILE=/opt/linaro/gcc-linaro-arm-linux-gnueabihf-4.8-2013.08_linux/bin/arm-linux-gnueabihf- + + or if you just installed gcc-arm-linux-gnueabi then it might be:: + + export CROSS_COMPILE=arm-linux-gnueabi- + +b. Configure and build U-Boot with verified boot enabled:: + + export UBOOT=/path/to/u-boot + cd $UBOOT + # You can add -j10 if you have 10 CPUs to make it faster + make O=b/am335x_boneblack_vboot am335x_boneblack_vboot_config all + export UOUT=$UBOOT/b/am335x_boneblack_vboot + +c. You will now have a U-Boot image:: + + file b/am335x_boneblack_vboot/u-boot-dtb.img + b/am335x_boneblack_vboot/u-boot-dtb.img: u-boot legacy uImage, + U-Boot 2014.07-rc2-00065-g2f69f8, Firmware/ARM, Firmware Image + (Not compressed), 395375 bytes, Sat May 31 16:19:04 2014, + Load Address: 0x80800000, Entry Point: 0x00000000, + Header CRC: 0x0ABD6ACA, Data CRC: 0x36DEF7E4 + + +Step 2: Build Linux +-------------------- + +a. Find the kernel image ('Image') and device tree (.dtb) file you plan to + use. In our case it is am335x-boneblack.dtb and it is built with the kernel. + At the time of writing an SD Boot image can be obtained from here:: + + http://www.elinux.org/Beagleboard:Updating_The_Software#Image_For_Booting_From_microSD + + You can write this to an SD card and then mount it to extract the kernel and + device tree files. + + You can also build a kernel. Instructions for this are are here:: + + http://elinux.org/Building_BBB_Kernel + + or you can use your favourite search engine. Following these instructions + produces a kernel Image and device tree files. For the record the steps + were:: + + export KERNEL=/path/to/kernel + cd $KERNEL + git clone git://github.com/beagleboard/kernel.git . + git checkout v3.14 + ./patch.sh + cp configs/beaglebone kernel/arch/arm/configs/beaglebone_defconfig + cd kernel + make beaglebone_defconfig + make uImage dtbs # -j10 if you have 10 CPUs + export OKERNEL=$KERNEL/kernel/arch/arm/boot + +b. You now have the 'Image' and 'am335x-boneblack.dtb' files needed to boot. + + +Step 3: Create the ITS +---------------------- + +Set up a directory for your work:: + + export WORK=/path/to/dir + cd $WORK + +Put this into a file in that directory called sign.its:: + + /dts-v1/; + + / { + description = "Beaglebone black"; + #address-cells = <1>; + + images { + kernel { + data = /incbin/("Image.lzo"); + type = "kernel"; + arch = "arm"; + os = "linux"; + compression = "lzo"; + load = <0x80008000>; + entry = <0x80008000>; + hash-1 { + algo = "sha1"; + }; + }; + fdt-1 { + description = "beaglebone-black"; + data = /incbin/("am335x-boneblack.dtb"); + type = "flat_dt"; + arch = "arm"; + compression = "none"; + hash-1 { + algo = "sha1"; + }; + }; + }; + configurations { + default = "conf-1"; + conf-1 { + kernel = "kernel"; + fdt = "fdt-1"; + signature-1 { + algo = "sha1,rsa2048"; + key-name-hint = "dev"; + sign-images = "fdt", "kernel"; + }; + }; + }; + }; + + +The explanation for this is all in the documentation you have already read. +But briefly it packages a kernel and device tree, and provides a single +configuration to be signed with a key named 'dev'. The kernel is compressed +with LZO to make it smaller. + + +Step 4: Create a key pair +------------------------- + +See :doc:`signature` for details on this step:: + + cd $WORK + mkdir keys + openssl genrsa -F4 -out keys/dev.key 2048 + openssl req -batch -new -x509 -key keys/dev.key -out keys/dev.crt + +Note: keys/dev.key contains your private key and is very secret. If anyone +gets access to that file they can sign kernels with it. Keep it secure. + + +Step 5: Sign the kernel +----------------------- + +We need to use mkimage (which was built when you built U-Boot) to package the +Linux kernel into a FIT (Flat Image Tree, a flexible file format that U-Boot +can load) using the ITS file you just created. + +At the same time we must put the public key into U-Boot device tree, with the +'required' property, which tells U-Boot that this key must be verified for the +image to be valid. You will make this key available to U-Boot for booting in +step 6:: + + ln -s $OKERNEL/dts/am335x-boneblack.dtb + ln -s $OKERNEL/Image + ln -s $UOUT/u-boot-dtb.img + cp $UOUT/arch/arm/dts/am335x-boneblack.dtb am335x-boneblack-pubkey.dtb + lzop Image + $UOUT/tools/mkimage -f sign.its -K am335x-boneblack-pubkey.dtb -k keys -r image.fit + +You should see something like this:: + + FIT description: Beaglebone black + Created: Sun Jun 1 12:50:30 2014 + Image 0 (kernel) + Description: unavailable + Created: Sun Jun 1 12:50:30 2014 + Type: Kernel Image + Compression: lzo compressed + Data Size: 7790938 Bytes = 7608.34 kB = 7.43 MB + Architecture: ARM + OS: Linux + Load Address: 0x80008000 + Entry Point: 0x80008000 + Hash algo: sha1 + Hash value: c94364646427e10f423837e559898ef02c97b988 + Image 1 (fdt-1) + Description: beaglebone-black + Created: Sun Jun 1 12:50:30 2014 + Type: Flat Device Tree + Compression: uncompressed + Data Size: 31547 Bytes = 30.81 kB = 0.03 MB + Architecture: ARM + Hash algo: sha1 + Hash value: cb09202f889d824f23b8e4404b781be5ad38a68d + Default Configuration: 'conf-1' + Configuration 0 (conf-1) + Description: unavailable + Kernel: kernel + FDT: fdt-1 + + +Now am335x-boneblack-pubkey.dtb contains the public key and image.fit contains +the signed kernel. Jump to step 6 if you like, or continue reading to increase +your understanding. + +You can also run fit_check_sign to check it:: + + $UOUT/tools/fit_check_sign -f image.fit -k am335x-boneblack-pubkey.dtb + +which results in:: + + Verifying Hash Integrity ... sha1,rsa2048:dev+ + ## Loading kernel from FIT Image at 7fc6ee469000 ... + Using 'conf-1' configuration + Verifying Hash Integrity ... + sha1,rsa2048:dev+ + OK + + Trying 'kernel' kernel subimage + Description: unavailable + Created: Sun Jun 1 12:50:30 2014 + Type: Kernel Image + Compression: lzo compressed + Data Size: 7790938 Bytes = 7608.34 kB = 7.43 MB + Architecture: ARM + OS: Linux + Load Address: 0x80008000 + Entry Point: 0x80008000 + Hash algo: sha1 + Hash value: c94364646427e10f423837e559898ef02c97b988 + Verifying Hash Integrity ... + sha1+ + OK + + Unimplemented compression type 4 + ## Loading fdt from FIT Image at 7fc6ee469000 ... + Using 'conf-1' configuration + Trying 'fdt-1' fdt subimage + Description: beaglebone-black + Created: Sun Jun 1 12:50:30 2014 + Type: Flat Device Tree + Compression: uncompressed + Data Size: 31547 Bytes = 30.81 kB = 0.03 MB + Architecture: ARM + Hash algo: sha1 + Hash value: cb09202f889d824f23b8e4404b781be5ad38a68d + Verifying Hash Integrity ... + sha1+ + OK + + Loading Flat Device Tree ... OK + + ## Loading ramdisk from FIT Image at 7fc6ee469000 ... + Using 'conf-1' configuration + Could not find subimage node + + Signature check OK + + +At the top, you see "sha1,rsa2048:dev+". This means that it checked an RSA key +of size 2048 bits using SHA1 as the hash algorithm. The key name checked was +'dev' and the '+' means that it verified. If it showed '-' that would be bad. + +Once the configuration is verified it is then possible to rely on the hashes +in each image referenced by that configuration. So fit_check_sign goes on to +load each of the images. We have a kernel and an FDT but no ramkdisk. In each +case fit_check_sign checks the hash and prints sha1+ meaning that the SHA1 +hash verified. This means that none of the images has been tampered with. + +There is a test in test/vboot which uses U-Boot's sandbox build to verify that +the above flow works. + +But it is fun to do this by hand, so you can load image.fit into a hex editor +like ghex, and change a byte in the kernel:: + + $UOUT/tools/fit_info -f image.fit -n /images/kernel -p data + NAME: kernel + LEN: 7790938 + OFF: 168 + +This tells us that the kernel starts at byte offset 168 (decimal) in image.fit +and extends for about 7MB. Try changing a byte at 0x2000 (say) and run +fit_check_sign again. You should see something like:: + + Verifying Hash Integrity ... sha1,rsa2048:dev+ + ## Loading kernel from FIT Image at 7f5a39571000 ... + Using 'conf-1' configuration + Verifying Hash Integrity ... + sha1,rsa2048:dev+ + OK + + Trying 'kernel' kernel subimage + Description: unavailable + Created: Sun Jun 1 13:09:21 2014 + Type: Kernel Image + Compression: lzo compressed + Data Size: 7790938 Bytes = 7608.34 kB = 7.43 MB + Architecture: ARM + OS: Linux + Load Address: 0x80008000 + Entry Point: 0x80008000 + Hash algo: sha1 + Hash value: c94364646427e10f423837e559898ef02c97b988 + Verifying Hash Integrity ... + sha1 error + Bad hash value for 'hash-1' hash node in 'kernel' image node + Bad Data Hash + + ## Loading fdt from FIT Image at 7f5a39571000 ... + Using 'conf-1' configuration + Trying 'fdt-1' fdt subimage + Description: beaglebone-black + Created: Sun Jun 1 13:09:21 2014 + Type: Flat Device Tree + Compression: uncompressed + Data Size: 31547 Bytes = 30.81 kB = 0.03 MB + Architecture: ARM + Hash algo: sha1 + Hash value: cb09202f889d824f23b8e4404b781be5ad38a68d + Verifying Hash Integrity ... + sha1+ + OK + + Loading Flat Device Tree ... OK + + ## Loading ramdisk from FIT Image at 7f5a39571000 ... + Using 'conf-1' configuration + Could not find subimage node + + Signature check Bad (error 1) + + +It has detected the change in the kernel. + +You can also be sneaky and try to switch images, using the libfdt utilities +that come with dtc (package name is device-tree-compiler but you will need a +recent version like 1.4:: + + dtc -v + Version: DTC 1.4.0 + +First we can check which nodes are actually hashed by the configuration:: + + $ fdtget -l image.fit / + images + configurations + + $ fdtget -l image.fit /configurations + conf-1 + fdtget -l image.fit /configurations/conf-1 + signature-1 + + $ fdtget -p image.fit /configurations/conf-1/signature-1 + hashed-strings + hashed-nodes + timestamp + signer-version + signer-name + value + algo + key-name-hint + sign-images + + $ fdtget image.fit /configurations/conf-1/signature-1 hashed-nodes + / /configurations/conf-1 /images/fdt-1 /images/fdt-1/hash /images/kernel /images/kernel/hash-1 + +This gives us a bit of a look into the signature that mkimage added. Note you +can also use fdtdump to list the entire device tree. + +Say we want to change the kernel that this configuration uses +(/images/kernel). We could just put a new kernel in the image, but we will +need to change the hash to match. Let's simulate that by changing a byte of +the hash:: + + fdtget -tx image.fit /images/kernel/hash-1 value + c9436464 6427e10f 423837e5 59898ef0 2c97b988 + fdtput -tx image.fit /images/kernel/hash-1 value c9436464 6427e10f 423837e5 59898ef0 2c97b981 + +Now check it again:: + + $UOUT/tools/fit_check_sign -f image.fit -k am335x-boneblack-pubkey.dtb + Verifying Hash Integrity ... sha1,rsa2048:devrsa_verify_with_keynode: RSA failed to verify: -13 + rsa_verify_with_keynode: RSA failed to verify: -13 + - + Failed to verify required signature 'key-dev' + Signature check Bad (error 1) + +This time we don't even get as far as checking the images, since the +configuration signature doesn't match. We can't change any hashes without the +signature check noticing. The configuration is essentially locked. U-Boot has +a public key for which it requires a match, and will not permit the use of any +configuration that does not match that public key. The only way the +configuration will match is if it was signed by the matching private key. + +It would also be possible to add a new signature node that does match your new +configuration. But that won't work since you are not allowed to change the +configuration in any way. Try it with a fresh (valid) image if you like by +running the mkimage link again. Then:: + + fdtput -p image.fit /configurations/conf-1/signature-1 value fred + $UOUT/tools/fit_check_sign -f image.fit -k am335x-boneblack-pubkey.dtb + Verifying Hash Integrity ... - + sha1,rsa2048:devrsa_verify_with_keynode: RSA failed to verify: -13 + rsa_verify_with_keynode: RSA failed to verify: -13 + - + Failed to verify required signature 'key-dev' + Signature check Bad (error 1) + + +Of course it would be possible to add an entirely new configuration and boot +with that, but it still needs to be signed, so it won't help. + + +6. Put the public key into U-Boot's image +----------------------------------------- + +Having confirmed that the signature is doing its job, let's try it out in +U-Boot on the board. U-Boot needs access to the public key corresponding to +the private key that you signed with so that it can verify any kernels that +you sign:: + + cd $UBOOT + make O=b/am335x_boneblack_vboot EXT_DTB=${WORK}/am335x-boneblack-pubkey.dtb + +Here we are overriding the normal device tree file with our one, which +contains the public key. + +Now you have a special U-Boot image with the public key. It can verify can +kernel that you sign with the private key as in step 5. + +If you like you can take a look at the public key information that mkimage +added to U-Boot's device tree:: + + fdtget -p am335x-boneblack-pubkey.dtb /signature/key-dev + required + algo + rsa,r-squared + rsa,modulus + rsa,n0-inverse + rsa,num-bits + key-name-hint + +This has information about the key and some pre-processed values which U-Boot +can use to verify against it. These values are obtained from the public key +certificate by mkimage, but require quite a bit of code to generate. To save +code space in U-Boot, the information is extracted and written in raw form for +U-Boot to easily use. The same mechanism is used in Google's Chrome OS. + +Notice the 'required' property. This marks the key as required - U-Boot will +not boot any image that does not verify against this key. + + +7. Put U-Boot and the kernel onto the board +------------------------------------------- + +The method here varies depending on how you are booting. For this example we +are booting from an micro-SD card with two partitions, one for U-Boot and one +for Linux. Put it into your machine and write U-Boot and the kernel to it. +Here the card is /dev/sde:: + + cd $WORK + export UDEV=/dev/sde1 # Change thes two lines to the correct device + export KDEV=/dev/sde2 + sudo mount $UDEV /mnt/tmp && sudo cp $UOUT/u-boot-dtb.img /mnt/tmp/u-boot.img && sleep 1 && sudo umount $UDEV + sudo mount $KDEV /mnt/tmp && sudo cp $WORK/image.fit /mnt/tmp/boot/image.fit && sleep 1 && sudo umount $KDEV + + +8. Try it +--------- + +Boot the board using the commands below:: + + setenv bootargs console=ttyO0,115200n8 quiet root=/dev/mmcblk0p2 ro rootfstype=ext4 rootwait + ext2load mmc 0:2 82000000 /boot/image.fit + bootm 82000000 + +You should then see something like this:: + + U-Boot# setenv bootargs console=ttyO0,115200n8 quiet root=/dev/mmcblk0p2 ro rootfstype=ext4 rootwait + U-Boot# ext2load mmc 0:2 82000000 /boot/image.fit + 7824930 bytes read in 589 ms (12.7 MiB/s) + U-Boot# bootm 82000000 + ## Loading kernel from FIT Image at 82000000 ... + Using 'conf-1' configuration + Verifying Hash Integrity ... sha1,rsa2048:dev+ OK + Trying 'kernel' kernel subimage + Description: unavailable + Created: 2014-06-01 19:32:54 UTC + Type: Kernel Image + Compression: lzo compressed + Data Start: 0x820000a8 + Data Size: 7790938 Bytes = 7.4 MiB + Architecture: ARM + OS: Linux + Load Address: 0x80008000 + Entry Point: 0x80008000 + Hash algo: sha1 + Hash value: c94364646427e10f423837e559898ef02c97b988 + Verifying Hash Integrity ... sha1+ OK + ## Loading fdt from FIT Image at 82000000 ... + Using 'conf-1' configuration + Trying 'fdt-1' fdt subimage + Description: beaglebone-black + Created: 2014-06-01 19:32:54 UTC + Type: Flat Device Tree + Compression: uncompressed + Data Start: 0x8276e2ec + Data Size: 31547 Bytes = 30.8 KiB + Architecture: ARM + Hash algo: sha1 + Hash value: cb09202f889d824f23b8e4404b781be5ad38a68d + Verifying Hash Integrity ... sha1+ OK + Booting using the fdt blob at 0x8276e2ec + Uncompressing Kernel Image ... OK + Loading Device Tree to 8fff5000, end 8ffffb3a ... OK + + Starting kernel ... + + [ 0.582377] omap_init_mbox: hwmod doesn't have valid attrs + [ 2.589651] musb-hdrc musb-hdrc.0.auto: Failed to request rx1. + [ 2.595830] musb-hdrc musb-hdrc.0.auto: musb_init_controller failed with status -517 + [ 2.606470] musb-hdrc musb-hdrc.1.auto: Failed to request rx1. + [ 2.612723] musb-hdrc musb-hdrc.1.auto: musb_init_controller failed with status -517 + [ 2.940808] drivers/rtc/hctosys.c: unable to open rtc device (rtc0) + [ 7.248889] libphy: PHY 4a101000.mdio:01 not found + [ 7.253995] net eth0: phy 4a101000.mdio:01 not found on slave 1 + systemd-fsck[83]: Angstrom: clean, 50607/218160 files, 306348/872448 blocks + + .---O---. + | | .-. o o + | | |-----.-----.-----.| | .----..-----.-----. + | | | __ | ---'| '--.| .-'| | | + | | | | | |--- || --'| | | ' | | | | + '---'---'--'--'--. |-----''----''--' '-----'-'-'-' + -' | + '---' + + The Angstrom Distribution beaglebone ttyO0 + + Angstrom v2012.12 - Kernel 3.14.1+ + + beaglebone login: + +At this point your kernel has been verified and you can be sure that it is one +that you signed. As an exercise, try changing image.fit as in step 5 and see +what happens. + + +Further Improvements +-------------------- + +Several of the steps here can be easily automated. In particular it would be +capital if signing and packaging a kernel were easy, perhaps a simple make +target in the kernel. + +Some mention of how to use multiple .dtb files in a FIT might be useful. + +U-Boot's verified boot mechanism has not had a robust and independent security +review. Such a review should look at the implementation and its resistance to +attacks. + +Perhaps the verified boot feature could be integrated into the Amstrom +distribution. + + +.. sectionauthor:: Simon Glass <sjg@chromium.org>, 2-June-14 diff --git a/doc/usage/fit/index.rst b/doc/usage/fit/index.rst index 0635d06b811..0576675ec00 100644 --- a/doc/usage/fit/index.rst +++ b/doc/usage/fit/index.rst @@ -12,3 +12,6 @@ doc/uImage.FIT source_file_format x86-fit-boot + signature + verified-boot + beaglebone_vboot diff --git a/doc/usage/fit/signature.rst b/doc/usage/fit/signature.rst new file mode 100644 index 00000000000..7d8292ece86 --- /dev/null +++ b/doc/usage/fit/signature.rst @@ -0,0 +1,760 @@ +.. SPDX-License-Identifier: GPL-2.0+ + +U-Boot FIT Signature Verification +================================= + +Introduction +------------ + +FIT supports hashing of images so that these hashes can be checked on +loading. This protects against corruption of the image. However it does not +prevent the substitution of one image for another. + +The signature feature allows the hash to be signed with a private key such +that it can be verified using a public key later. Provided that the private +key is kept secret and the public key is stored in a non-volatile place, +any image can be verified in this way. + +See verified-boot.txt for more general information on verified boot. + + +Concepts +-------- + +Some familiarity with public key cryptography is assumed in this section. + +The procedure for signing is as follows: + + - hash an image in the FIT + - sign the hash with a private key to produce a signature + - store the resulting signature in the FIT + +The procedure for verification is: + + - read the FIT + - obtain the public key + - extract the signature from the FIT + - hash the image from the FIT + - verify (with the public key) that the extracted signature matches the + hash + +The signing is generally performed by mkimage, as part of making a firmware +image for the device. The verification is normally done in U-Boot on the +device. + + +Algorithms +---------- +In principle any suitable algorithm can be used to sign and verify a hash. +U-Boot supports a few hashing and verification algorithms. See below for +details. + +While it is acceptable to bring in large cryptographic libraries such as +openssl on the host side (e.g. mkimage), it is not desirable for U-Boot. +For the run-time verification side, it is important to keep code and data +size as small as possible. + +For this reason the RSA image verification uses pre-processed public keys +which can be used with a very small amount of code - just some extraction +of data from the FDT and exponentiation mod n. Code size impact is a little +under 5KB on Tegra Seaboard, for example. + +It is relatively straightforward to add new algorithms if required. If +another RSA variant is needed, then it can be added with the +U_BOOT_CRYPTO_ALGO() macro. If another algorithm is needed (such as DSA) then +it can be placed in a directory alongside lib/rsa/, and its functions added +using U_BOOT_CRYPTO_ALGO(). + + +Creating an RSA key pair and certificate +---------------------------------------- +To create a new public/private key pair, size 2048 bits:: + + $ openssl genpkey -algorithm RSA -out keys/dev.key \ + -pkeyopt rsa_keygen_bits:2048 -pkeyopt rsa_keygen_pubexp:65537 + +To create a certificate for this containing the public key:: + + $ openssl req -batch -new -x509 -key keys/dev.key -out keys/dev.crt + +If you like you can look at the public key also:: + + $ openssl rsa -in keys/dev.key -pubout + + +Device Tree Bindings +-------------------- +The following properties are required in the FIT's signature node(s) to +allow the signer to operate. These should be added to the .its file. +Signature nodes sit at the same level as hash nodes and are called +signature-1, signature-2, etc. + +algo + Algorithm name (e.g. "sha1,rsa2048") + +key-name-hint + Name of key to use for signing. The keys will normally be in + a single directory (parameter -k to mkimage). For a given key <name>, its + private key is stored in <name>.key and the certificate is stored in + <name>.crt. + +When the image is signed, the following properties are added (mandatory): + +value + The signature data (e.g. 256 bytes for 2048-bit RSA) + +When the image is signed, the following properties are optional: + +timestamp + Time when image was signed (standard Unix time_t format) + +signer-name + Name of the signer (e.g. "mkimage") + +signer-version + Version string of the signer (e.g. "2013.01") + +comment + Additional information about the signer or image + +padding + The padding algorithm, it may be pkcs-1.5 or pss, + if no value is provided we assume pkcs-1.5 + +For config bindings (see Signed Configurations below), the following +additional properties are optional: + +sign-images + A list of images to sign, each being a property of the conf + node that contains then. The default is "kernel,fdt" which means that these + two images will be looked up in the config and signed if present. + +For config bindings, these properties are added by the signer: + +hashed-nodes + A list of nodes which were hashed by the signer. Each is + a string - the full path to node. A typical value might be:: + + hashed-nodes = "/", "/configurations/conf-1", "/images/kernel", + "/images/kernel/hash-1", "/images/fdt-1", + "/images/fdt-1/hash-1"; + +hashed-strings + The start and size of the string region of the FIT that was hashed + +Example: See :doc:`sign-images` for an example image tree source file and +sign-configs.its for config signing. + + +Public Key Storage +------------------ +In order to verify an image that has been signed with a public key we need to +have a trusted public key. This cannot be stored in the signed image, since +it would be easy to alter. For this implementation we choose to store the +public key in U-Boot's control FDT (using CONFIG_OF_CONTROL). + +Public keys should be stored as sub-nodes in a /signature node. Required +properties are: + +algo + Algorithm name (e.g. "sha1,rsa2048" or "sha256,ecdsa256") + +Optional properties are: + +key-name-hint + Name of key used for signing. This is only a hint since it + is possible for the name to be changed. Verification can proceed by checking + all available signing keys until one matches. + +required + If present this indicates that the key must be verified for the + image / configuration to be considered valid. Only required keys are + normally verified by the FIT image booting algorithm. Valid values are + "image" to force verification of all images, and "conf" to force verification + of the selected configuration (which then relies on hashes in the images to + verify those). + +Each signing algorithm has its own additional properties. + +For RSA the following are mandatory: + +rsa,num-bits + Number of key bits (e.g. 2048) + +rsa,modulus + Modulus (N) as a big-endian multi-word integer + +rsa,exponent + Public exponent (E) as a 64 bit unsigned integer + +rsa,r-squared + (2^num-bits)^2 as a big-endian multi-word integer + +rsa,n0-inverse + -1 / modulus[0] mod 2^32 + +For ECDSA the following are mandatory: + +ecdsa,curve + Name of ECDSA curve (e.g. "prime256v1") + +ecdsa,x-point + Public key X coordinate as a big-endian multi-word integer + +ecdsa,y-point + Public key Y coordinate as a big-endian multi-word integer + +These parameters can be added to a binary device tree using parameter -K of the +mkimage command:: + + tools/mkimage -f fit.its -K control.dtb -k keys -r image.fit + +Here is an example of a generated device tree node:: + + signature { + key-dev { + required = "conf"; + algo = "sha256,rsa2048"; + rsa,r-squared = <0xb76d1acf 0xa1763ca5 0xeb2f126 + 0x742edc80 0xd3f42177 0x9741d9d9 + 0x35bb476e 0xff41c718 0xd3801430 + 0xf22537cb 0xa7e79960 0xae32a043 + 0x7da1427a 0x341d6492 0x3c2762f5 + 0xaac04726 0x5b262d96 0xf984e86d + 0xb99443c7 0x17080c33 0x940f6892 + 0xd57a95d1 0x6ea7b691 0xc5038fa8 + 0x6bb48a6e 0x73f1b1ea 0x37160841 + 0xe05715ce 0xa7c45bbd 0x690d82d5 + 0x99c2454c 0x6ff117b3 0xd830683b + 0x3f81c9cf 0x1ca38a91 0x0c3392e4 + 0xd817c625 0x7b8e9a24 0x175b89ea + 0xad79f3dc 0x4d50d7b4 0x9d4e90f8 + 0xad9e2939 0xc165d6a4 0x0ada7e1b + 0xfb1bf495 0xfc3131c2 0xb8c6e604 + 0xc2761124 0xf63de4a6 0x0e9565f9 + 0xc8e53761 0x7e7a37a5 0xe99dcdae + 0x9aff7e1e 0xbd44b13d 0x6b0e6aa4 + 0x038907e4 0x8e0d6850 0xef51bc20 + 0xf73c94af 0x88bea7b1 0xcbbb1b30 + 0xd024b7f3>; + rsa,modulus = <0xc0711d6cb 0x9e86db7f 0x45986dbe + 0x023f1e8c9 0xe1a4c4d0 0x8a0dfdc9 + 0x023ba0c48 0x06815f6a 0x5caa0654 + 0x07078c4b7 0x3d154853 0x40729023 + 0x0b007c8fe 0x5a3647e5 0x23b41e20 + 0x024720591 0x66915305 0x0e0b29b0 + 0x0de2ad30d 0x8589430f 0xb1590325 + 0x0fb9f5d5e 0x9eba752a 0xd88e6de9 + 0x056b3dcc6 0x9a6b8e61 0x6784f61f + 0x000f39c21 0x5eec6b33 0xd78e4f78 + 0x0921a305f 0xaa2cc27e 0x1ca917af + 0x06e1134f4 0xd48cac77 0x4e914d07 + 0x0f707aa5a 0x0d141f41 0x84677f1d + 0x0ad47a049 0x028aedb6 0xd5536fcf + 0x03fef1e4f 0x133a03d2 0xfd7a750a + 0x0f9159732 0xd207812e 0x6a807375 + 0x06434230d 0xc8e22dad 0x9f29b3d6 + 0x07c44ac2b 0xfa2aad88 0xe2429504 + 0x041febd41 0x85d0d142 0x7b194d65 + 0x06e5d55ea 0x41116961 0xf3181dde + 0x068bf5fbc 0x3dd82047 0x00ee647e + 0x0d7a44ab3>; + rsa,exponent = <0x00 0x10001>; + rsa,n0-inverse = <0xb3928b85>; + rsa,num-bits = <0x800>; + key-name-hint = "dev"; + }; + }; + + +Signed Configurations +--------------------- +While signing images is useful, it does not provide complete protection +against several types of attack. For example, it is possible to create a +FIT with the same signed images, but with the configuration changed such +that a different one is selected (mix and match attack). It is also possible +to substitute a signed image from an older FIT version into a newer FIT +(roll-back attack). + +As an example, consider this FIT:: + + / { + images { + kernel-1 { + data = <data for kernel1> + signature-1 { + algo = "sha1,rsa2048"; + value = <...kernel signature 1...> + }; + }; + kernel-2 { + data = <data for kernel2> + signature-1 { + algo = "sha1,rsa2048"; + value = <...kernel signature 2...> + }; + }; + fdt-1 { + data = <data for fdt1>; + signature-1 { + algo = "sha1,rsa2048"; + value = <...fdt signature 1...> + }; + }; + fdt-2 { + data = <data for fdt2>; + signature-1 { + algo = "sha1,rsa2048"; + value = <...fdt signature 2...> + }; + }; + }; + configurations { + default = "conf-1"; + conf-1 { + kernel = "kernel-1"; + fdt = "fdt-1"; + }; + conf-2 { + kernel = "kernel-2"; + fdt = "fdt-2"; + }; + }; + }; + +Since both kernels are signed it is easy for an attacker to add a new +configuration 3 with kernel 1 and fdt 2:: + + configurations { + default = "conf-1"; + conf-1 { + kernel = "kernel-1"; + fdt = "fdt-1"; + }; + conf-2 { + kernel = "kernel-2"; + fdt = "fdt-2"; + }; + conf-3 { + kernel = "kernel-1"; + fdt = "fdt-2"; + }; + }; + +With signed images, nothing protects against this. Whether it gains an +advantage for the attacker is debatable, but it is not secure. + +To solve this problem, we support signed configurations. In this case it +is the configurations that are signed, not the image. Each image has its +own hash, and we include the hash in the configuration signature. + +So the above example is adjusted to look like this:: + + / { + images { + kernel-1 { + data = <data for kernel1> + hash-1 { + algo = "sha1"; + value = <...kernel hash 1...> + }; + }; + kernel-2 { + data = <data for kernel2> + hash-1 { + algo = "sha1"; + value = <...kernel hash 2...> + }; + }; + fdt-1 { + data = <data for fdt1>; + hash-1 { + algo = "sha1"; + value = <...fdt hash 1...> + }; + }; + fdt-2 { + data = <data for fdt2>; + hash-1 { + algo = "sha1"; + value = <...fdt hash 2...> + }; + }; + }; + configurations { + default = "conf-1"; + conf-1 { + kernel = "kernel-1"; + fdt = "fdt-1"; + signature-1 { + algo = "sha1,rsa2048"; + value = <...conf 1 signature...>; + }; + }; + conf-2 { + kernel = "kernel-2"; + fdt = "fdt-2"; + signature-1 { + algo = "sha1,rsa2048"; + value = <...conf 1 signature...>; + }; + }; + }; + }; + + +You can see that we have added hashes for all images (since they are no +longer signed), and a signature to each configuration. In the above example, +mkimage will sign configurations/conf-1, the kernel and fdt that are +pointed to by the configuration (/images/kernel-1, /images/kernel-1/hash-1, +/images/fdt-1, /images/fdt-1/hash-1) and the root structure of the image +(so that it isn't possible to add or remove root nodes). The signature is +written into /configurations/conf-1/signature-1/value. It can easily be +verified later even if the FIT has been signed with other keys in the +meantime. + + +Details +------- +The signature node contains a property ('hashed-nodes') which lists all the +nodes that the signature was made over. The image is walked in order and each +tag processed as follows: + +DTB_BEGIN_NODE + The tag and the following name are included in the signature + if the node or its parent are present in 'hashed-nodes' + +DTB_END_NODE + The tag is included in the signature if the node or its parent + are present in 'hashed-nodes' + +DTB_PROPERTY + The tag, the length word, the offset in the string table, and + the data are all included if the current node is present in 'hashed-nodes' + and the property name is not 'data'. + +DTB_END + The tag is always included in the signature. + +DTB_NOP + The tag is included in the signature if the current node is present + in 'hashed-nodes' + +In addition, the signature contains a property 'hashed-strings' which contains +the offset and length in the string table of the strings that are to be +included in the signature (this is done last). + +IMPORTANT: To verify the signature outside u-boot, it is vital to not only +calculate the hash of the image and verify the signature with that, but also to +calculate the hashes of the kernel, fdt, and ramdisk images and check those +match the hash values in the corresponding 'hash*' subnodes. + + +Verification +------------ +FITs are verified when loaded. After the configuration is selected a list +of required images is produced. If there are 'required' public keys, then +each image must be verified against those keys. This means that every image +that might be used by the target needs to be signed with 'required' keys. + +This happens automatically as part of a bootm command when FITs are used. + +For Signed Configurations, the default verification behavior can be changed by +the following optional property in /signature node in U-Boot's control FDT. + +required-mode + Valid values are "any" to allow verified boot to succeed if + the selected configuration is signed by any of the 'required' keys, and "all" + to allow verified boot to succeed if the selected configuration is signed by + all of the 'required' keys. + +This property can be added to a binary device tree using fdtput as shown in +below examples:: + + fdtput -t s control.dtb /signature required-mode any + fdtput -t s control.dtb /signature required-mode all + + +Enabling FIT Verification +------------------------- +In addition to the options to enable FIT itself, the following CONFIGs must +be enabled: + +CONFIG_FIT_SIGNATURE + enable signing and verification in FITs + +CONFIG_RSA + enable RSA algorithm for signing + +CONFIG_ECDSA + enable ECDSA algorithm for signing + +WARNING: When relying on signed FIT images with required signature check +the legacy image format is default disabled by not defining +CONFIG_LEGACY_IMAGE_FORMAT + + +Testing +------- + +An easy way to test signing and verification is to use the test script +provided in test/vboot/vboot_test.sh. This uses sandbox (a special version +of U-Boot which runs under Linux) to show the operation of a 'bootm' +command loading and verifying images. + +A sample run is show below:: + + $ make O=sandbox sandbox_config + $ make O=sandbox + $ O=sandbox ./test/vboot/vboot_test.sh + + +Simple Verified Boot Test +------------------------- + +Please see :doc:`verified-boot` for more information:: + + /home/hs/ids/u-boot/sandbox/tools/mkimage -D -I dts -O dtb -p 2000 + Build keys + do sha1 test + Build FIT with signed images + Test Verified Boot Run: unsigned signatures:: OK + Sign images + Test Verified Boot Run: signed images: OK + Build FIT with signed configuration + Test Verified Boot Run: unsigned config: OK + Sign images + Test Verified Boot Run: signed config: OK + check signed config on the host + Signature check OK + OK + Test Verified Boot Run: signed config: OK + Test Verified Boot Run: signed config with bad hash: OK + do sha256 test + Build FIT with signed images + Test Verified Boot Run: unsigned signatures:: OK + Sign images + Test Verified Boot Run: signed images: OK + Build FIT with signed configuration + Test Verified Boot Run: unsigned config: OK + Sign images + Test Verified Boot Run: signed config: OK + check signed config on the host + Signature check OK + OK + Test Verified Boot Run: signed config: OK + Test Verified Boot Run: signed config with bad hash: OK + + Test passed + + +Software signing: keydir vs keyfile +----------------------------------- + +In the simplest case, signing is done by giving mkimage the 'keyfile'. This is +the path to a file containing the signing key. + +The alternative is to pass the 'keydir' argument. In this case the filename of +the key is derived from the 'keydir' and the "key-name-hint" property in the +FIT. In this case the "key-name-hint" property is mandatory, and the key must +exist in "<keydir>/<key-name-hint>.<ext>" Here the extension "ext" is +specific to the signing algorithm. + + +Hardware Signing with PKCS#11 or with HSM +----------------------------------------- + +Securely managing private signing keys can challenging, especially when the +keys are stored on the file system of a computer that is connected to the +Internet. If an attacker is able to steal the key, they can sign malicious FIT +images which will appear genuine to your devices. + +An alternative solution is to keep your signing key securely stored on hardware +device like a smartcard, USB token or Hardware Security Module (HSM) and have +them perform the signing. PKCS#11 is standard for interfacing with these crypto +device. + +Requirements: + - Smartcard/USB token/HSM which can work with some openssl engine + - openssl + +For pkcs11 engine usage: + - libp11 (provides pkcs11 engine) + - p11-kit (recommended to simplify setup) + - opensc (for smartcards and smartcard like USB devices) + - gnutls (recommended for key generation, p11tool) + +For generic HSMs respective openssl engine must be installed and locateable by +openssl. This may require setting up LD_LIBRARY_PATH if engine is not installed +to openssl's default search paths. + +PKCS11 engine support forms "key id" based on "keydir" and with +"key-name-hint". "key-name-hint" is used as "object" name (if not defined in +keydir). "keydir" (if defined) is used to define (prefix for) which PKCS11 source +is being used for lookup up for the key. + +PKCS11 engine key ids + "pkcs11:<keydir>;object=<key-name-hint>;type=<public|private>" + +or, if keydir contains "object=" + "pkcs11:<keydir>;type=<public|private>" + +or + "pkcs11:object=<key-name-hint>;type=<public|private>", + +Generic HSM engine support forms "key id" based on "keydir" and with +"key-name-hint". If "keydir" is specified for mkimage it is used as a prefix in +"key id" and is appended with "key-name-hint". + +Generic engine key ids: + "<keydir><key-name-hint>" + +or + "< key-name-hint>" + +In order to set the pin in the HSM, an environment variable "MKIMAGE_SIGN_PIN" +can be specified. + +The following examples use the Nitrokey Pro using pkcs11 engine. Instructions +for other devices may vary. + +Notes on pkcs11 engine setup: + +Make sure p11-kit, opensc are installed and that p11-kit is setup to use opensc. +/usr/share/p11-kit/modules/opensc.module should be present on your system. + + +Generating Keys On the Nitrokey:: + + $ gpg --card-edit + + Reader ...........: Nitrokey Nitrokey Pro (xxxxxxxx0000000000000000) 00 00 + Application ID ...: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx + Version ..........: 2.1 + Manufacturer .....: ZeitControl + Serial number ....: xxxxxxxx + Name of cardholder: [not set] + Language prefs ...: de + Sex ..............: unspecified + URL of public key : [not set] + Login data .......: [not set] + Signature PIN ....: forced + Key attributes ...: rsa2048 rsa2048 rsa2048 + Max. PIN lengths .: 32 32 32 + PIN retry counter : 3 0 3 + Signature counter : 0 + Signature key ....: [none] + Encryption key....: [none] + Authentication key: [none] + General key info..: [none] + + gpg/card> generate + Make off-card backup of encryption key? (Y/n) n + + Please note that the factory settings of the PINs are + PIN = '123456' Admin PIN = '12345678' + You should change them using the command --change-pin + + What keysize do you want for the Signature key? (2048) 4096 + The card will now be re-configured to generate a key of 4096 bits + Note: There is no guarantee that the card supports the requested size. + If the key generation does not succeed, please check the + documentation of your card to see what sizes are allowed. + What keysize do you want for the Encryption key? (2048) 4096 + The card will now be re-configured to generate a key of 4096 bits + What keysize do you want for the Authentication key? (2048) 4096 + The card will now be re-configured to generate a key of 4096 bits + Please specify how long the key should be valid. + 0 = key does not expire + <n> = key expires in n days + <n>w = key expires in n weeks + <n>m = key expires in n months + <n>y = key expires in n years + Key is valid for? (0) + Key does not expire at all + Is this correct? (y/N) y + + GnuPG needs to construct a user ID to identify your key. + + Real name: John Doe + Email address: john.doe@email.com + Comment: + You selected this USER-ID: + "John Doe <john.doe@email.com>" + + Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? o + + +Using p11tool to get the token URL: + +Depending on system configuration, gpg-agent may need to be killed first:: + + $ p11tool --provider /usr/lib/opensc-pkcs11.so --list-tokens + Token 0: + URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29 + Label: OpenPGP card (User PIN (sig)) + Type: Hardware token + Manufacturer: ZeitControl + Model: PKCS#15 emulated + Serial: 000xxxxxxxxx + Module: (null) + + + Token 1: + URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%29 + Label: OpenPGP card (User PIN) + Type: Hardware token + Manufacturer: ZeitControl + Model: PKCS#15 emulated + Serial: 000xxxxxxxxx + Module: (null) + +Use the portion of the signature token URL after "pkcs11:" as the keydir argument (-k) to mkimage below. + + +Use the URL of the token to list the private keys:: + + $ p11tool --login --provider /usr/lib/opensc-pkcs11.so --list-privkeys \ + "pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29" + Token 'OpenPGP card (User PIN (sig))' with URL 'pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29' requires user PIN + Enter PIN: + Object 0: + URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29;id=%01;object=Signature%20key;type=private + Type: Private key + Label: Signature key + Flags: CKA_PRIVATE; CKA_NEVER_EXTRACTABLE; CKA_SENSITIVE; + ID: 01 + +Use the label, in this case "Signature key" as the key-name-hint in your FIT. + +Create the fitImage:: + + $ ./tools/mkimage -f fit-image.its fitImage + + +Sign the fitImage with the hardware key:: + + $ ./tools/mkimage -F -k \ + "model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29" \ + -K u-boot.dtb -N pkcs11 -r fitImage + + +Future Work +----------- + +- Roll-back protection using a TPM is done using the tpm command. This can + be scripted, but we might consider a default way of doing this, built into + bootm. + + +Possible Future Work +-------------------- + +- More sandbox tests for failure modes +- Passwords for keys/certificates +- Perhaps implement OAEP +- Enhance bootm to permit scripted signature verification (so that a script + can verify an image but not actually boot it) + + +.. sectionauthor:: Simon Glass <sjg@chromium.org>, 1-1-13 diff --git a/doc/uImage.FIT/verified-boot.txt b/doc/usage/fit/verified-boot.rst index 41c9fa9e09f..301207711db 100644 --- a/doc/uImage.FIT/verified-boot.txt +++ b/doc/usage/fit/verified-boot.rst @@ -1,8 +1,11 @@ +.. SPDX-License-Identifier: GPL-2.0+ + U-Boot Verified Boot ==================== Introduction ------------ + Verified boot here means the verification of all software loaded into a machine during the boot process to ensure that it is authorised and correct for that machine. @@ -21,6 +24,7 @@ memory, so that firmware can easily be upgraded in a secure manner. Signing ------- + Verified boot uses cryptographic algorithms to 'sign' software images. Images are signed using a private key known only to the signer, but can be verified using a public key. As its name suggests the public key can be @@ -28,31 +32,31 @@ made available without risk to the verification process. The private and public keys are mathematically related. For more information on how this works look up "public key cryptography" and "RSA" (a particular algorithm). -The signing and verification process looks something like this: - - - Signing Verification - ======= ============ - - +--------------+ * - | RSA key pair | * +---------------+ - | .key .crt | * | Public key in | - +--------------+ +------> public key ----->| trusted place | - | | * +---------------+ - | | * | - v | * v - +---------+ | * +--------------+ - | |----------+ * | | - | signer | * | U-Boot | - | |----------+ * | signature |--> yes/no - +---------+ | * | verification | - ^ | * | | - | | * +--------------+ - | | * ^ - +----------+ | * | - | Software | +----> signed image -------------+ - | image | * - +----------+ * +The signing and verification process looks something like this:: + + + Signing Verification + ======= ============ + + +--------------+ * + | RSA key pair | * +---------------+ + | .key .crt | * | Public key in | + +--------------+ +------> public key ----->| trusted place | + | | * +---------------+ + | | * | + v | * v + +---------+ | * +--------------+ + | |---------+ * | | + | signer | * | U-Boot | + | |---------+ * | signature |--> yes/no + +---------+ | * | verification | + ^ | * | | + | | * +--------------+ + | | * ^ + +----------+ | * | + | Software | +----> signed image -------------+ + | image | * + +----------+ * The signature algorithm relies only on the public key to do its work. Using @@ -70,23 +74,25 @@ the verification is worthless. Chaining Images --------------- + The above method works for a signer providing images to a run-time U-Boot. It is also possible to extend this scheme to a second level, like this: -1. Master private key is used by the signer to sign a first-stage image. -2. Master public key is placed in read-only memory. -2. Secondary private key is created and used to sign second-stage images. -3. Secondary public key is placed in first stage images -4. We use the master public key to verify the first-stage image. We then -use the secondary public key in the first-stage image to verify the second- -state image. -5. This chaining process can go on indefinitely. It is recommended to use a -different key at each stage, so that a compromise in one place will not -affect the whole change. +#. Master private key is used by the signer to sign a first-stage image. +#. Master public key is placed in read-only memory. +#. Secondary private key is created and used to sign second-stage images. +#. Secondary public key is placed in first stage images +#. We use the master public key to verify the first-stage image. We then + use the secondary public key in the first-stage image to verify the second- + state image. +#. This chaining process can go on indefinitely. It is recommended to use a + different key at each stage, so that a compromise in one place will not + affect the whole change. Flattened Image Tree (FIT) -------------------------- + The FIT format is already widely used in U-Boot. It is a flattened device tree (FDT) in a particular format, with images contained within. FITs include hashes to verify images, so it is relatively straightforward to @@ -96,9 +102,6 @@ The public key can be stored in U-Boot's CONFIG_OF_CONTROL device tree in a standard place. Then when a FIT is loaded it can be verified using that public key. Multiple keys and multiple signatures are supported. -See signature.txt for more information. - +See :doc:`signature` for more information. -Simon Glass -sjg@chromium.org -1-1-13 +.. sectionauthor:: Simon Glass <sjg@chromium.org> 1-1-13 |