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

[Peter Zijlstra <peterz@infradead.org>]

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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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