mm: rmap: fix use-after-free in __put_anon_vma

While working address sanitizer for kernel I've discovered
use-after-free bug in __put_anon_vma.

For the last anon_vma, anon_vma->root freed before child anon_vma.
Later in anon_vma_free(anon_vma) we are referencing to already freed
anon_vma->root to check rwsem.

This fixes it by freeing the child anon_vma before freeing
anon_vma->root.

Signed-off-by: Andrey Ryabinin <a.ryabinin@samsung.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: <stable@vger.kernel.org> # v3.0+
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 2 deletions

diff --git a/mm/rmap.c b/mm/rmap.c
index 9c3e77396d1a..83bfafabb47b 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1564,10 +1564,9 @@ void __put_anon_vma(struct anon_vma *anon_vma)
 {
         struct anon_vma *root = anon_vma->root;
 
+        anon_vma_free(anon_vma);
         if (root != anon_vma && atomic_dec_and_test(&root->refcount))
                 anon_vma_free(root);
-
-        anon_vma_free(anon_vma);
 }
 
 static struct anon_vma *rmap_walk_anon_lock(struct page *page,