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Commit 4837fe37 authored by Michal Hocko's avatar Michal Hocko Committed by Linus Torvalds
Browse files

mm, oom_reaper: fix memory corruption 

David Rientjes has reported the following memory corruption while the
oom reaper tries to unmap the victims address space

  BUG: Bad page map in process oom_reaper  pte:6353826300000000 pmd:00000000
  addr:00007f50cab1d000 vm_flags:08100073 anon_vma:ffff9eea335603f0 mapping:          (null) index:7f50cab1d
  file:          (null) fault:          (null) mmap:          (null) readpage:          (null)
  CPU: 2 PID: 1001 Comm: oom_reaper
  Call Trace:
     unmap_page_range+0x1068/0x1130
     __oom_reap_task_mm+0xd5/0x16b
     oom_reaper+0xff/0x14c
     kthread+0xc1/0xe0

Tetsuo Handa has noticed that the synchronization inside exit_mmap is
insufficient.  We only synchronize with the oom reaper if
tsk_is_oom_victim which is not true if the final __mmput is called from
a different context than the oom victim exit path.  This can trivially
happen from context of any task which has grabbed mm reference (e.g.  to
read /proc/<pid>/ file which requires mm etc.).

The race would look like this

  oom_reaper		oom_victim		task
						mmget_not_zero
			do_exit
			  mmput
  __oom_reap_task_mm				mmput
  						  __mmput
						    exit_mmap
						      remove_vma
    unmap_page_range

Fix this issue by providing a new mm_is_oom_victim() helper which
operates on the mm struct rather than a task.  Any context which
operates on a remote mm struct should use this helper in place of
tsk_is_oom_victim.  The flag is set in mark_oom_victim and never cleared
so it is stable in the exit_mmap path.

Debugged by Tetsuo Handa.

Link: http://lkml.kernel.org/r/20171210095130.17110-1-mhocko@kernel.org


Fixes: 21292580 ("mm: oom: let oom_reap_task and exit_mmap run concurrently")
Signed-off-by: default avatarMichal Hocko <mhocko@suse.com>
Reported-by: default avatarDavid Rientjes <rientjes@google.com>
Acked-by: default avatarDavid Rientjes <rientjes@google.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Andrea Argangeli <andrea@kernel.org>
Cc: <stable@vger.kernel.org>	[4.14]
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent bdcf0a42

include/linux/oom.h

+9 −0
Original line number Diff line number Diff line
@@ -66,6 +66,15 @@ static inline bool tsk_is_oom_victim(struct task_struct * tsk) 
	return tsk->signal->oom_mm;

}



/*

 * Use this helper if tsk->mm != mm and the victim mm needs a special

 * handling. This is guaranteed to stay true after once set.

 */

static inline bool mm_is_oom_victim(struct mm_struct *mm)

{

	return test_bit(MMF_OOM_VICTIM, &mm->flags);

}



/*

 * Checks whether a page fault on the given mm is still reliable.

 * This is no longer true if the oom reaper started to reap the

include/linux/sched/coredump.h

+1 −0
Original line number Diff line number Diff line
@@ -70,6 +70,7 @@ static inline int get_dumpable(struct mm_struct *mm) 
#define MMF_UNSTABLE		22	/* mm is unstable for copy_from_user */

#define MMF_HUGE_ZERO_PAGE	23      /* mm has ever used the global huge zero page */

#define MMF_DISABLE_THP		24	/* disable THP for all VMAs */

#define MMF_OOM_VICTIM		25	/* mm is the oom victim */

#define MMF_DISABLE_THP_MASK	(1 << MMF_DISABLE_THP)



#define MMF_INIT_MASK		(MMF_DUMPABLE_MASK | MMF_DUMP_FILTER_MASK |\

mm/mmap.c

+5 −5
Original line number Diff line number Diff line
@@ -3019,20 +3019,20 @@ void exit_mmap(struct mm_struct *mm) 
	/* Use -1 here to ensure all VMAs in the mm are unmapped */

	unmap_vmas(&tlb, vma, 0, -1);



	set_bit(MMF_OOM_SKIP, &mm->flags);

	if (unlikely(tsk_is_oom_victim(current))) {

	if (unlikely(mm_is_oom_victim(mm))) {

		/*

		 * Wait for oom_reap_task() to stop working on this

		 * mm. Because MMF_OOM_SKIP is already set before

		 * calling down_read(), oom_reap_task() will not run

		 * on this "mm" post up_write().

		 *

		 * tsk_is_oom_victim() cannot be set from under us

		 * either because current->mm is already set to NULL

		 * mm_is_oom_victim() cannot be set from under us

		 * either because victim->mm is already set to NULL

		 * under task_lock before calling mmput and oom_mm is

		 * set not NULL by the OOM killer only if current->mm

		 * set not NULL by the OOM killer only if victim->mm

		 * is found not NULL while holding the task_lock.

		 */

		set_bit(MMF_OOM_SKIP, &mm->flags);

		down_write(&mm->mmap_sem);

		up_write(&mm->mmap_sem);

	}

mm/oom_kill.c

+3 −1
Original line number Diff line number Diff line
@@ -683,8 +683,10 @@ static void mark_oom_victim(struct task_struct *tsk) 
		return;



	/* oom_mm is bound to the signal struct life time. */

	if (!cmpxchg(&tsk->signal->oom_mm, NULL, mm))

	if (!cmpxchg(&tsk->signal->oom_mm, NULL, mm)) {

		mmgrab(tsk->signal->oom_mm);

		set_bit(MMF_OOM_VICTIM, &mm->flags);

	}



	/*

	 * Make sure that the task is woken up from uninterruptible sleep