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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/[email protected]/ [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/[email protected] Link: https://lkml.kernel.org/r/[email protected] Signed-off-by: Daniel Colascione <[email protected]> Signed-off-by: Lokesh Gidra <[email protected]> Reviewed-by: Andrea Arcangeli <[email protected]> Cc: Alexander Viro <[email protected]> Cc: <[email protected]> Cc: Daniel Colascione <[email protected]> Cc: Eric Biggers <[email protected]> Cc: Iurii Zaikin <[email protected]> Cc: Jeff Vander Stoep <[email protected]> Cc: Jerome Glisse <[email protected]> Cc: "Joel Fernandes (Google)" <[email protected]> Cc: Johannes Weiner <[email protected]> Cc: Jonathan Corbet <[email protected]> Cc: Kalesh Singh <[email protected]> Cc: Kees Cook <[email protected]> Cc: Luis Chamberlain <[email protected]> Cc: Mauro Carvalho Chehab <[email protected]> Cc: Mel Gorman <[email protected]> Cc: Mike Rapoport <[email protected]> Cc: Nitin Gupta <[email protected]> Cc: Peter Xu <[email protected]> Cc: Sebastian Andrzej Siewior <[email protected]> Cc: Shaohua Li <[email protected]> Cc: Stephen Smalley <[email protected]> Cc: Suren Baghdasaryan <[email protected]> Cc: Vlastimil Babka <[email protected]> Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: Linus Torvalds <[email protected]>
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