Re: [PATCH 02/13] mm: Revalidate anon_vma in page_lock_anon_vma()

Nick Piggin
2010-04-09T03:16:47Z

On Thu, Apr 08, 2010 at 09:17:39PM +0200, Peter Zijlstra wrote:
> There is nothing preventing the anon_vma from being detached while we
> are spinning to acquire the lock. Most (all?) current users end up
> calling something like vma_address(page, vma) on it, which has a
> fairly good chance of weeding out wonky vmas.
>
> However suppose the anon_vma got freed and re-used while we were
> waiting to acquire the lock, and the new anon_vma fits with the
> page->index (because that is the only thing vma_address() uses to
> determine if the page fits in a particular vma, we could end up
> traversing faulty anon_vma chains.
>
> Close this hole for good by re-validating that page->mapping still
> holds the very same anon_vma pointer after we acquire the lock, if not
> be utterly paranoid and retry the whole operation (which will very
> likely bail, because it's unlikely the page got attached to a different
> anon_vma in the meantime).

Hm, looks like a bugfix? How was this supposed to be safe?

> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> ---
> mm/rmap.c | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> Index: linux-2.6/mm/rmap.c
> ===================================================================
> --- linux-2.6.orig/mm/rmap.c
> +++ linux-2.6/mm/rmap.c
> @@ -294,6 +294,7 @@ struct anon_vma *page_lock_anon_vma(stru
> unsigned long anon_mapping;
>
> rcu_read_lock();
> +again:
> anon_mapping = (unsigned long) ACCESS_ONCE(page->mapping);
> if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON)
> goto out;
> @@ -302,6 +303,12 @@ struct anon_vma *page_lock_anon_vma(stru
>
> anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
> spin_lock(&anon_vma->lock);
> +
> + if (page_rmapping(page) != anon_vma) {

very unlikely()?

> + spin_unlock(&anon_vma->lock);
> + goto again;
> + }
> +
> return anon_vma;
> out:
> rcu_read_unlock();
>