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authorLokesh Gidra2020-12-14 19:13:49 -0800
committerLinus Torvalds2020-12-15 12:13:46 -0800
commit37cd0575b8510159992d279c530c05f872990b02 (patch)
tree46e0d999c81821045cbe0d923d6c4c95d5ebf6c0
parentf289041ed4cf9a3f6e8a32068fef9ffb2acc5662 (diff)
userfaultfd: add UFFD_USER_MODE_ONLY
Patch series "Control over userfaultfd kernel-fault handling", v6. This patch series is split from [1]. The other series enables SELinux support for userfaultfd file descriptors so that its creation and movement can be controlled. It has been demonstrated on various occasions that suspending kernel code execution for an arbitrary amount of time at any access to userspace memory (copy_from_user()/copy_to_user()/...) can be exploited to change the intended behavior of the kernel. For instance, handling page faults in kernel-mode using userfaultfd has been exploited in [2, 3]. Likewise, FUSE, which is similar to userfaultfd in this respect, has been exploited in [4, 5] for similar outcome. This small patch series adds a new flag to userfaultfd(2) that allows callers to give up the ability to handle kernel-mode faults with the resulting UFFD file object. It then adds a 'user-mode only' option to the unprivileged_userfaultfd sysctl knob to require unprivileged callers to use this new flag. The purpose of this new interface is to decrease the chance of an unprivileged userfaultfd user taking advantage of userfaultfd to enhance security vulnerabilities by lengthening the race window in kernel code. [1] https://lore.kernel.org/lkml/20200211225547.235083-1-dancol@google.com/ [2] https://duasynt.com/blog/linux-kernel-heap-spray [3] https://duasynt.com/blog/cve-2016-6187-heap-off-by-one-exploit [4] https://googleprojectzero.blogspot.com/2016/06/exploiting-recursion-in-linux-kernel_20.html [5] https://bugs.chromium.org/p/project-zero/issues/detail?id=808 This patch (of 2): userfaultfd handles page faults from both user and kernel code. Add a new UFFD_USER_MODE_ONLY flag for userfaultfd(2) that makes the resulting userfaultfd object refuse to handle faults from kernel mode, treating these faults as if SIGBUS were always raised, causing the kernel code to fail with EFAULT. A future patch adds a knob allowing administrators to give some processes the ability to create userfaultfd file objects only if they pass UFFD_USER_MODE_ONLY, reducing the likelihood that these processes will exploit userfaultfd's ability to delay kernel page faults to open timing windows for future exploits. Link: https://lkml.kernel.org/r/20201120030411.2690816-1-lokeshgidra@google.com Link: https://lkml.kernel.org/r/20201120030411.2690816-2-lokeshgidra@google.com Signed-off-by: Daniel Colascione <dancol@google.com> Signed-off-by: Lokesh Gidra <lokeshgidra@google.com> Reviewed-by: Andrea Arcangeli <aarcange@redhat.com> Cc: Alexander Viro <viro@zeniv.linux.org.uk> Cc: <calin@google.com> Cc: Daniel Colascione <dancol@dancol.org> Cc: Eric Biggers <ebiggers@kernel.org> Cc: Iurii Zaikin <yzaikin@google.com> Cc: Jeff Vander Stoep <jeffv@google.com> Cc: Jerome Glisse <jglisse@redhat.com> Cc: "Joel Fernandes (Google)" <joel@joelfernandes.org> Cc: Johannes Weiner <hannes@cmpxchg.org> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Kalesh Singh <kaleshsingh@google.com> Cc: Kees Cook <keescook@chromium.org> Cc: Luis Chamberlain <mcgrof@kernel.org> Cc: Mauro Carvalho Chehab <mchehab+huawei@kernel.org> Cc: Mel Gorman <mgorman@techsingularity.net> Cc: Mike Rapoport <rppt@linux.vnet.ibm.com> Cc: Nitin Gupta <nigupta@nvidia.com> Cc: Peter Xu <peterx@redhat.com> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Cc: Shaohua Li <shli@fb.com> Cc: Stephen Smalley <stephen.smalley.work@gmail.com> Cc: Suren Baghdasaryan <surenb@google.com> Cc: Vlastimil Babka <vbabka@suse.cz> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--fs/userfaultfd.c10
-rw-r--r--include/uapi/linux/userfaultfd.h9
2 files changed, 18 insertions, 1 deletions
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 000b457ad087..605599fde015 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -405,6 +405,13 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
if (ctx->features & UFFD_FEATURE_SIGBUS)
goto out;
+ if ((vmf->flags & FAULT_FLAG_USER) == 0 &&
+ ctx->flags & UFFD_USER_MODE_ONLY) {
+ printk_once(KERN_WARNING "uffd: Set unprivileged_userfaultfd "
+ "sysctl knob to 1 if kernel faults must be handled "
+ "without obtaining CAP_SYS_PTRACE capability\n");
+ goto out;
+ }
/*
* If it's already released don't get it. This avoids to loop
@@ -1965,10 +1972,11 @@ SYSCALL_DEFINE1(userfaultfd, int, flags)
BUG_ON(!current->mm);
/* Check the UFFD_* constants for consistency. */
+ BUILD_BUG_ON(UFFD_USER_MODE_ONLY & UFFD_SHARED_FCNTL_FLAGS);
BUILD_BUG_ON(UFFD_CLOEXEC != O_CLOEXEC);
BUILD_BUG_ON(UFFD_NONBLOCK != O_NONBLOCK);
- if (flags & ~UFFD_SHARED_FCNTL_FLAGS)
+ if (flags & ~(UFFD_SHARED_FCNTL_FLAGS | UFFD_USER_MODE_ONLY))
return -EINVAL;
ctx = kmem_cache_alloc(userfaultfd_ctx_cachep, GFP_KERNEL);
diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
index e7e98bde221f..5f2d88212f7c 100644
--- a/include/uapi/linux/userfaultfd.h
+++ b/include/uapi/linux/userfaultfd.h
@@ -257,4 +257,13 @@ struct uffdio_writeprotect {
__u64 mode;
};
+/*
+ * Flags for the userfaultfd(2) system call itself.
+ */
+
+/*
+ * Create a userfaultfd that can handle page faults only in user mode.
+ */
+#define UFFD_USER_MODE_ONLY 1
+
#endif /* _LINUX_USERFAULTFD_H */