linux-kernel - Re: [PATCH -v2] rmap: make anon_vma_prepare link in all the anon

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <1271100621.20295.3397.camel@laptop>
Date:	Mon, 12 Apr 2010 21:30:21 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Rik van Riel <riel@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Johannes Weiner <hannes@...xchg.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Nick Piggin <npiggin@...e.de>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	sgunderson@...foot.com
Subject: Re: [PATCH -v2] rmap: make anon_vma_prepare link in all the
 anon_vmas of a mergeable VMA

On Mon, 2010-04-12 at 20:40 +0200, Peter Zijlstra wrote:

Hmm, if interleaved like so

> struct anon_vma *page_lock_anon_vma(struct page *page)
> {
>         struct anon_vma *anon_vma;
>         unsigned long anon_mapping;

page_remove_rmap()
anon_vma_unlink()
  anon_vma_free()

So that the below will all observe the old page->mapping:

>         rcu_read_lock();
> again:
>         anon_mapping = (unsigned long)rcu_dereference(page->mapping);
>         if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON)
>                 goto out;
>         anon_vma = (struct anon_vma *)(anon_mapping - PAGE_MAPPING_ANON);
> 
>         /*
>          * The RCU read lock ensures we can safely dereference anon_vma
>          * since it ensures the backing slab won't go away. It will however
>          * not guarantee it's the right object.
>          *
>          * First take the anon_vma->lock, this will, per anon_vma_unlink()
>          * avoid this anon_vma from being freed if it is a valid object.
>          */
>         spin_lock(&anon_vma->lock);
> 
>         /*
>          * Secondly, we have to re-read page->mapping, so ensure it
>          * has not changed, rely on spin_lock() being at least a
>          * compiler barrier to force the re-read.
>          */
>         if (unlikely(page_rmapping(page) != anon_vma)) {
>                 spin_unlock(&anon_vma->lock);
>                 goto again;
>         }

page_add_anon_rmap(), so that the page_mapped() test below would be
positive,

>         /*
>          * Ensure we read page->mapping before page->_mapcount,
>          * orders against atomic_add_negative() in page_remove_rmap().
>          */
>         smp_rmb();
> 
>         /*
>          * Finally check that the page is still mapped,
>          * if not, this can't possibly be the right anon_vma.
>          */
>         if (!page_mapped(page))
>                 goto unlock;

We could here return a non-valid and already freed anon_vma.

>         return anon_vma;
> 
> unlock:
>         spin_unlock(&anon_vma->lock);
> out:
>         rcu_read_unlock();
>         return NULL;
> }
> 
> 

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/