Commits
Michal Hocko committed 3f70dc38cec
mm: make sure that kthreads will not refault oom reaped memory There are only few use_mm() users in the kernel right now. Most of them write to the target memory but vhost driver relies on copy_from_user/get_user from a kernel thread context. This makes it impossible to reap the memory of an oom victim which shares the mm with the vhost kernel thread because it could see a zero page unexpectedly and theoretically make an incorrect decision visible outside of the killed task context. To quote Michael S. Tsirkin: : Getting an error from __get_user and friends is handled gracefully. : Getting zero instead of a real value will cause userspace : memory corruption. The vhost kernel thread is bound to an open fd of the vhost device which is not tight to the mm owner life cycle in general. The device fd can be inherited or passed over to another process which means that we really have to be careful about unexpected memory corruption because unlike for normal oom victims the result will be visible outside of the oom victim context. Make sure that no kthread context (users of use_mm) can ever see corrupted data because of the oom reaper and hook into the page fault path by checking MMF_UNSTABLE mm flag. __oom_reap_task_mm will set the flag before it starts unmapping the address space while the flag is checked after the page fault has been handled. If the flag is set then SIGBUS is triggered so any g-u-p user will get a error code. Regular tasks do not need this protection because all which share the mm are killed when the mm is reaped and so the corruption will not outlive them. This patch shouldn't have any visible effect at this moment because the OOM killer doesn't invoke oom reaper for tasks with mm shared with kthreads yet. Link: http://lkml.kernel.org/r/1472119394-11342-9-git-send-email-mhocko@kernel.org Signed-off-by: Michal Hocko <mhocko@suse.com> Acked-by: "Michael S. Tsirkin" <mst@redhat.com> Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> Cc: Oleg Nesterov <oleg@redhat.com> Cc: David Rientjes <rientjes@google.com> Cc: Vladimir Davydov <vdavydov@parallels.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>