[PATCH] mm: rmap: fix use-after-free in __put_anon_vma

[Andrey Ryabinin]

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 patch puts freeing of child anon_vma before freeing of anon_vma->root.

Cc: stable@xxxxxxxxxxxxxxx # v3.0+
Signed-off-by: Andrey Ryabinin <a.ryabinin@xxxxxxxxxxx>
---
 mm/rmap.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 9c3e773..161bffc7 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1564,10 +1564,11 @@ void __put_anon_vma(struct anon_vma *anon_vma)
 {
 	struct anon_vma *root = anon_vma->root;
 
-	if (root != anon_vma && atomic_dec_and_test(&root->refcount))
+	if (root != anon_vma && atomic_dec_and_test(&root->refcount)) {
+		anon_vma_free(anon_vma);
 		anon_vma_free(root);
-
-	anon_vma_free(anon_vma);
+	} else
+		anon_vma_free(anon_vma);
 }
 
 static struct anon_vma *rmap_walk_anon_lock(struct page *page,
-- 
1.8.5.5

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