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

Peter Zijlstra
2014-06-06T11:56:26Z

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On Fri, Jun 06, 2014 at 03:30:55PM +0400, Andrey Ryabinin wrote:
> 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.

Yes, I think that is right indeed.

Very hard to hit, but valid since not all callers hold rcu_read_lock().

>
> Cc: stable@vger.kernel.org # v3.0+
> Signed-off-by: Andrey Ryabinin <a.ryabinin@samsung.com>
> ---
> 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);
> }

Why not simply move the freeing of anon_vma before the root, like:

anon_vma_free(anon_vma);
if (root != anon_vma && atomic_dec_and_test(&root->refcount))
anon_vma_free(root);

?

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