mm: fix potential anon_vma locking issue in mprotect()

Fix an anon_vma locking issue in the following situation:

- vma has no anon_vma
- next has an anon_vma
- vma is being shrunk / next is being expanded, due to an mprotect call

We need to take next's anon_vma lock to avoid races with rmap users (such
as page migration) while next is being expanded.

Signed-off-by: Michel Lespinasse <walken@google.com>
Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>
Acked-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 1 deletions

diff --git a/mm/mmap.c b/mm/mmap.c
index 2ba27434e843..e3c365ff1b6a 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -573,8 +573,12 @@ again:			remove_next = 1 + (end > next->vm_end);
          */
         if (vma->anon_vma && (importer || start != vma->vm_start)) {
                 anon_vma = vma->anon_vma;
+                VM_BUG_ON(adjust_next && next->anon_vma &&
+                          anon_vma != next->anon_vma);
+        } else if (adjust_next && next->anon_vma)
+                anon_vma = next->anon_vma;
+        if (anon_vma)
                 anon_vma_lock(anon_vma);
-        }
 
         if (root) {
                 flush_dcache_mmap_lock(mapping);