linux-kernel - Re: Ugly rmap NULL ptr deref oopsie on hibernate (was Linux 2.6.34-rc3)

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Date:	Wed, 07 Apr 2010 00:58:54 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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>
Subject: Re: Ugly rmap NULL ptr deref oopsie on hibernate (was Linux
 2.6.34-rc3)

On Tue, 2010-04-06 at 11:40 -0400, Rik van Riel wrote:
> On 04/06/2010 11:34 AM, Minchan Kim wrote:
> 
> > Let's see the unlink_anon_vmas.
> >
> > 1. list_for_each_entry_safe(avc,next, vma->anon_vma_chain, same_vma)
> > 2. 	anon_vma_unlink
> > 3. 		spin_lock(anon_vma->lock)<-- HERE LOCK.
> > 4.		list_del(anon_vma_chain->same_anon_vma);
> >
> > What if anon_vma is destroyed and reuse by SLAB_XXX_RCU for another
> > anon_vma object between 2 and 3?
> > I mean how to make sure 3) does lock valid anon_vma?
> >
> > I hope it is culprit.
> 
> How can the anon_vma get destroyed and reused, when this
> anon_vma_chain still has a reference to it (and the

Doesn't anon_vma_chain have a ref counter on anon_vma?

> anon_vma has not been freed yet)?

AFAIK, anon_vma can be reused without free by SLAB_XXX_RCU.
So we always use it carefully by page_lock_anon_vma or manual check
with RCU and page_mapped. 

What am I missing?

> 
> What combination of circumstances is necessary for
> your bug hypothetical to happen?


	CPU A				CPU B

unlink_anon_vmas
list_for_each_entry 
				
					free_pgtable
					anon_vma_unlink
  <crazy stall>				spin_lock(anon_vma);
					list_del(same_anon_vma)
					spin_unlock(anon_vma)
anon_vma_unlink
					anon_vma_free
					reuse for another anon_vma
spin_lock(another anon_vma)
list_del(another anon_vma)

If my assumption is wrong, please correct me. 
Thanks, Rik. 

-- 
Kind regards,
Minchan Kim


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