mm, oom: fix concurrent munlock and oom reaper unmap, v3

Since exit_mmap() is done without the protection of mm->mmap_sem, it is
possible for the oom reaper to concurrently operate on an mm until
MMF_OOM_SKIP is set.

This allows munlock_vma_pages_all() to concurrently run while the oom
reaper is operating on a vma.  Since munlock_vma_pages_range() depends
on clearing VM_LOCKED from vm_flags before actually doing the munlock to
determine if any other vmas are locking the same memory, the check for
VM_LOCKED in the oom reaper is racy.

This is especially noticeable on architectures such as powerpc where
clearing a huge pmd requires serialize_against_pte_lookup().  If the pmd
is zapped by the oom reaper during follow_page_mask() after the check
for pmd_none() is bypassed, this ends up deferencing a NULL ptl or a
kernel oops.

Fix this by manually freeing all possible memory from the mm before
doing the munlock and then setting MMF_OOM_SKIP.  The oom reaper can not
run on the mm anymore so the munlock is safe to do in exit_mmap().  It
also matches the logic that the oom reaper currently uses for
determining when to set MMF_OOM_SKIP itself, so there's no new risk of
excessive oom killing.

This issue fixes CVE-2018-1000200.

Link: http://lkml.kernel.org/r/alpine.DEB.2.21.1804241526320.238665@chino.kir.corp.google.com
Fixes: 212925802454 ("mm: oom: let oom_reap_task and exit_mmap run concurrently")
Signed-off-by: David Rientjes <rientjes@google.com>
Suggested-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
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>

3 files changed, 71 insertions, 56 deletions

diff --git a/include/linux/oom.h b/include/linux/oom.h
index 5bad038ac012..6adac113e96d 100644
--- a/include/linux/oom.h
+++ b/include/linux/oom.h
@@ -95,6 +95,8 @@ static inline int check_stable_address_space(struct mm_struct *mm)
         return 0;
 }
 
+void __oom_reap_task_mm(struct mm_struct *mm);
+
 extern unsigned long oom_badness(struct task_struct *p,
                 struct mem_cgroup *memcg, const nodemask_t *nodemask,
                 unsigned long totalpages);
diff --git a/mm/mmap.c b/mm/mmap.c
index 9d5968d1e8e3..d6836566e4e5 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -3024,6 +3024,32 @@ void exit_mmap(struct mm_struct *mm)
         /* mm's last user has gone, and its about to be pulled down */
         mmu_notifier_release(mm);
 
+        if (unlikely(mm_is_oom_victim(mm))) {
+                /*
+                 * Manually reap the mm to free as much memory as possible.
+                 * Then, as the oom reaper does, set MMF_OOM_SKIP to disregard
+                 * this mm from further consideration.  Taking mm->mmap_sem for
+                 * write after setting MMF_OOM_SKIP will guarantee that the oom
+                 * reaper will not run on this mm again after mmap_sem is
+                 * dropped.
+                 *
+                 * Nothing can be holding mm->mmap_sem here and the above call
+                 * to mmu_notifier_release(mm) ensures mmu notifier callbacks in
+                 * __oom_reap_task_mm() will not block.
+                 *
+                 * This needs to be done before calling munlock_vma_pages_all(),
+                 * which clears VM_LOCKED, otherwise the oom reaper cannot
+                 * reliably test it.
+                 */
+                mutex_lock(&oom_lock);
+                __oom_reap_task_mm(mm);
+                mutex_unlock(&oom_lock);
+
+                set_bit(MMF_OOM_SKIP, &mm->flags);
+                down_write(&mm->mmap_sem);
+                up_write(&mm->mmap_sem);
+        }
+
         if (mm->locked_vm) {
                 vma = mm->mmap;
                 while (vma) {
@@ -3045,24 +3071,6 @@ void exit_mmap(struct mm_struct *mm)
         /* update_hiwater_rss(mm) here? but nobody should be looking */
         /* Use -1 here to ensure all VMAs in the mm are unmapped */
         unmap_vmas(&tlb, vma, 0, -1);
-
-        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().
-                 *
-                 * 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 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);
-        }
         free_pgtables(&tlb, vma, FIRST_USER_ADDRESS, USER_PGTABLES_CEILING);
         tlb_finish_mmu(&tlb, 0, -1);
 
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index ff992fa8760a..8ba6cb88cf58 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -469,7 +469,6 @@ bool process_shares_mm(struct task_struct *p, struct mm_struct *mm)
         return false;
 }
 
-
 #ifdef CONFIG_MMU
 /*
  * OOM Reaper kernel thread which tries to reap the memory used by the OOM
@@ -480,16 +479,54 @@ static DECLARE_WAIT_QUEUE_HEAD(oom_reaper_wait);
 static struct task_struct *oom_reaper_list;
 static DEFINE_SPINLOCK(oom_reaper_lock);
 
-static bool __oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
+void __oom_reap_task_mm(struct mm_struct *mm)
 {
-        struct mmu_gather tlb;
         struct vm_area_struct *vma;
+
+        /*
+         * Tell all users of get_user/copy_from_user etc... that the content
+         * is no longer stable. No barriers really needed because unmapping
+         * should imply barriers already and the reader would hit a page fault
+         * if it stumbled over a reaped memory.
+         */
+        set_bit(MMF_UNSTABLE, &mm->flags);
+
+        for (vma = mm->mmap ; vma; vma = vma->vm_next) {
+                if (!can_madv_dontneed_vma(vma))
+                        continue;
+
+                /*
+                 * Only anonymous pages have a good chance to be dropped
+                 * without additional steps which we cannot afford as we
+                 * are OOM already.
+                 *
+                 * We do not even care about fs backed pages because all
+                 * which are reclaimable have already been reclaimed and
+                 * we do not want to block exit_mmap by keeping mm ref
+                 * count elevated without a good reason.
+                 */
+                if (vma_is_anonymous(vma) || !(vma->vm_flags & VM_SHARED)) {
+                        const unsigned long start = vma->vm_start;
+                        const unsigned long end = vma->vm_end;
+                        struct mmu_gather tlb;
+
+                        tlb_gather_mmu(&tlb, mm, start, end);
+                        mmu_notifier_invalidate_range_start(mm, start, end);
+                        unmap_page_range(&tlb, vma, start, end, NULL);
+                        mmu_notifier_invalidate_range_end(mm, start, end);
+                        tlb_finish_mmu(&tlb, start, end);
+                }
+        }
+}
+
+static bool oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
+{
         bool ret = true;
 
         /*
          * We have to make sure to not race with the victim exit path
          * and cause premature new oom victim selection:
-         * __oom_reap_task_mm           exit_mm
+         * oom_reap_task_mm             exit_mm
          *   mmget_not_zero
          *                                mmput
          *                                  atomic_dec_and_test
@@ -534,39 +571,8 @@ static bool __oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
 
         trace_start_task_reaping(tsk->pid);
 
-        /*
-         * Tell all users of get_user/copy_from_user etc... that the content
-         * is no longer stable. No barriers really needed because unmapping
-         * should imply barriers already and the reader would hit a page fault
-         * if it stumbled over a reaped memory.
-         */
-        set_bit(MMF_UNSTABLE, &mm->flags);
-
-        for (vma = mm->mmap ; vma; vma = vma->vm_next) {
-                if (!can_madv_dontneed_vma(vma))
-                        continue;
+        __oom_reap_task_mm(mm);
 
-                /*
-                 * Only anonymous pages have a good chance to be dropped
-                 * without additional steps which we cannot afford as we
-                 * are OOM already.
-                 *
-                 * We do not even care about fs backed pages because all
-                 * which are reclaimable have already been reclaimed and
-                 * we do not want to block exit_mmap by keeping mm ref
-                 * count elevated without a good reason.
-                 */
-                if (vma_is_anonymous(vma) || !(vma->vm_flags & VM_SHARED)) {
-                        const unsigned long start = vma->vm_start;
-                        const unsigned long end = vma->vm_end;
-
-                        tlb_gather_mmu(&tlb, mm, start, end);
-                        mmu_notifier_invalidate_range_start(mm, start, end);
-                        unmap_page_range(&tlb, vma, start, end, NULL);
-                        mmu_notifier_invalidate_range_end(mm, start, end);
-                        tlb_finish_mmu(&tlb, start, end);
-                }
-        }
         pr_info("oom_reaper: reaped process %d (%s), now anon-rss:%lukB, file-rss:%lukB, shmem-rss:%lukB\n",
                         task_pid_nr(tsk), tsk->comm,
                         K(get_mm_counter(mm, MM_ANONPAGES)),
@@ -587,14 +593,13 @@ static void oom_reap_task(struct task_struct *tsk)
         struct mm_struct *mm = tsk->signal->oom_mm;
 
         /* Retry the down_read_trylock(mmap_sem) a few times */
-        while (attempts++ < MAX_OOM_REAP_RETRIES && !__oom_reap_task_mm(tsk, mm))
+        while (attempts++ < MAX_OOM_REAP_RETRIES && !oom_reap_task_mm(tsk, mm))
                 schedule_timeout_idle(HZ/10);
 
         if (attempts <= MAX_OOM_REAP_RETRIES ||
             test_bit(MMF_OOM_SKIP, &mm->flags))
                 goto done;
 
-
         pr_info("oom_reaper: unable to reap pid:%d (%s)\n",
                 task_pid_nr(tsk), tsk->comm);
         debug_show_all_locks();