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/[e-mail]
Fixes: 212925802454 ("mm: oom: let oom_reap_task and exit_mmap run concurrently")
Signed-off-by: Michal Hocko <[e-mail]>
Reported-by: David Rientjes <[e-mail]>
Acked-by: David Rientjes <[e-mail]>
Cc: Tetsuo Handa <[e-mail]>
Cc: Andrea Argangeli <[e-mail]>
Cc: <[e-mail]> [4.14]
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/oom.h

···
 }
 
 /*
+ * 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
  * address space which is reflected by MMF_UNSTABLE flag set in

include/linux/sched/coredump.h

···
 #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

···
 	/* 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

···
 		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