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Diffstat (limited to 'Documentation/power/swsusp.txt')
| -rw-r--r-- | Documentation/power/swsusp.txt | 83 |
1 files changed, 81 insertions, 2 deletions
diff --git a/Documentation/power/swsusp.txt b/Documentation/power/swsusp.txt index 4e1627cc5b51..7a6b78966459 100644 --- a/Documentation/power/swsusp.txt +++ b/Documentation/power/swsusp.txt @@ -164,10 +164,11 @@ place where the thread is safe to be frozen (no kernel semaphores should be held at that point and it must be safe to sleep there), and add: - try_to_freeze(); + try_to_freeze(); If the thread is needed for writing the image to storage, you should -instead set the PF_NOFREEZE process flag when creating the thread. +instead set the PF_NOFREEZE process flag when creating the thread (and +be very carefull). Q: What is the difference between between "platform", "shutdown" and @@ -232,3 +233,81 @@ A: Try running cat `cat /proc/[0-9]*/maps | grep / | sed 's:.* /:/:' | sort -u` > /dev/null after resume. swapoff -a; swapon -a may also be usefull. + +Q: What happens to devices during swsusp? They seem to be resumed +during system suspend? + +A: That's correct. We need to resume them if we want to write image to +disk. Whole sequence goes like + + Suspend part + ~~~~~~~~~~~~ + running system, user asks for suspend-to-disk + + user processes are stopped + + suspend(PMSG_FREEZE): devices are frozen so that they don't interfere + with state snapshot + + state snapshot: copy of whole used memory is taken with interrupts disabled + + resume(): devices are woken up so that we can write image to swap + + write image to swap + + suspend(PMSG_SUSPEND): suspend devices so that we can power off + + turn the power off + + Resume part + ~~~~~~~~~~~ + (is actually pretty similar) + + running system, user asks for suspend-to-disk + + user processes are stopped (in common case there are none, but with resume-from-initrd, noone knows) + + read image from disk + + suspend(PMSG_FREEZE): devices are frozen so that they don't interfere + with image restoration + + image restoration: rewrite memory with image + + resume(): devices are woken up so that system can continue + + thaw all user processes + +Q: What is this 'Encrypt suspend image' for? + +A: First of all: it is not a replacement for dm-crypt encrypted swap. +It cannot protect your computer while it is suspended. Instead it does +protect from leaking sensitive data after resume from suspend. + +Think of the following: you suspend while an application is running +that keeps sensitive data in memory. The application itself prevents +the data from being swapped out. Suspend, however, must write these +data to swap to be able to resume later on. Without suspend encryption +your sensitive data are then stored in plaintext on disk. This means +that after resume your sensitive data are accessible to all +applications having direct access to the swap device which was used +for suspend. If you don't need swap after resume these data can remain +on disk virtually forever. Thus it can happen that your system gets +broken in weeks later and sensitive data which you thought were +encrypted and protected are retrieved and stolen from the swap device. +To prevent this situation you should use 'Encrypt suspend image'. + +During suspend a temporary key is created and this key is used to +encrypt the data written to disk. When, during resume, the data was +read back into memory the temporary key is destroyed which simply +means that all data written to disk during suspend are then +inaccessible so they can't be stolen later on. The only thing that +you must then take care of is that you call 'mkswap' for the swap +partition used for suspend as early as possible during regular +boot. This asserts that any temporary key from an oopsed suspend or +from a failed or aborted resume is erased from the swap device. + +As a rule of thumb use encrypted swap to protect your data while your +system is shut down or suspended. Additionally use the encrypted +suspend image to prevent sensitive data from being stolen after +resume. |