linux-kernel - Re: [BUG 3.0.0-rc1] ksm: NULL pointer dereference in ksm_do

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Date:	Fri, 3 Jun 2011 20:13:22 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Chris Wright <chrisw@...s-sol.org>,
	Andrea Righi <andrea@...terlinux.com>,
	CAI Qian <caiqian@...hat.com>, Rik van Riel <riel@...hat.com>,
	Mel Gorman <mel@....ul.ie>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [BUG 3.0.0-rc1] ksm: NULL pointer dereference in ksm_do_scan()

On Fri, Jun 03, 2011 at 10:06:14AM -0700, Hugh Dickins wrote:
> On Thu, 2 Jun 2011, Andrea Arcangeli wrote:
> > On Thu, Jun 02, 2011 at 10:29:39AM -0700, Hugh Dickins wrote:
> > > AndreaA, I didn't study the patch you posted half an hour ago,
> > > since by that time I'd worked it out and was preparing patch below.
> > > I think your patch would be for a different bug, hopefully one we
> > > don't have, it looks more complicated than we should need for this.
> > 
> > I didn't expect two different bugs leading to double free.
> 
> There wasn't a double free there, just failure to cope with race
> emptying the list, so accessing head when expecting a full entry.

Yes, we thought it was a double free initially because of two dead
pointers but we couldn't explain why mm was null so consistently.

> You'll see from the "beware" comment in scan_get_next_rmap_item()
> that this case is expected, that it sometimes reaches freeing the
> slots before the exiting task reaches __ksm_exit().
> 
> That race should already be handled.  I believe your patch is unnecessary,
> because get_mm_slot() is a hashlist lookup, and will return NULL once
> either end has done the hlist_del(&mm_slot->link).

Ok so that case is handled by get_mm_slot not succeeding. I see thanks
for the review.
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