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: 212925802454 ("mm: oom: let oom_reap_task and exit_mmap run concurrently")
Signed-off-by: Michal Hocko <mhocko@suse.com>
Reported-by: David Rientjes <rientjes@google.com>
Acked-by: David 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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

4 files changed, 18 insertions, 6 deletions

diff --git a/include/linux/oom.h b/include/linux/oom.h
index 01c91d874a57..5bad038ac012 100644
--- a/include/linux/oom.h
+++ b/include/linux/oom.h
@@ -67,6 +67,15 @@ static inline bool tsk_is_oom_victim(struct task_struct * tsk)
 }
 
 /*
+ * 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
diff --git a/include/linux/sched/coredump.h b/include/linux/sched/coredump.h
index 9c8847395b5e..ec912d01126f 100644
--- a/include/linux/sched/coredump.h
+++ b/include/linux/sched/coredump.h
@@ -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 |\
diff --git a/mm/mmap.c b/mm/mmap.c
index a4d546821214..9efdc021ad22 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -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);
         }
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index c957be32b27a..29f855551efe 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -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