linux-kernel - Re: [stable] please do not merge 058504edd026 (was Re: fs: use after free in /proc/pid/mountinfo)

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Date:	Mon, 7 Jul 2014 16:06:27 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Greg KH <greg@...ah.com>
Cc:	David Rientjes <rientjes@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>, stable@...r.kernel.org,
	Al Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>,
	Dave Jones <davej@...hat.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [stable] please do not merge 058504edd026 (was Re: fs: use
 after free in /proc/pid/mountinfo)

On Mon, 7 Jul 2014 16:05:42 -0700 Greg KH <greg@...ah.com> wrote:

> On Fri, Jul 04, 2014 at 03:35:54PM -0700, David Rientjes wrote:
> > On Fri, 4 Jul 2014, Sasha Levin wrote:
> > 
> > > > Does this now reproduce on Linus's tree?  If so, does reverting commit
> > > > 058504edd026 ("fs/seq_file: fallback to vmalloc allocation") prevent this 
> > > > issue?
> > > > 
> > > > This is a use-after-free since the poison value is 0x6b and I'm presuming 
> > > > that your /proc/self/mountinfo may be larger than PAGE_SIZE in your 
> > > > testing environment.
> > > > 
> > > 
> > > Good call, reverting that patch made both issues go away.
> > > 
> > 
> > Thanks for checking, Sasha.
> > 
> > Stable maintainers, please do not merge commit 058504edd026 ("fs/seq_file: 
> > fallback to vmalloc allocation") that is annotated with a cc for 
> > stable@...r.kernel.org in Linus's tree into stable kernels.
> > 
> > Although we're at 3.16-rc3, I'm hoping that we can get a fix for the 
> > use-after-free in the next couple days before asking for a revert.  Sasha 
> > confirms[*] this commit causes the bug.
> 
> I should also drop "Subject: /proc/stat: convert to single_open_size()"
> from the -stable tree, right?
> 

That would be best.

I can't see how "fs: use after free in /proc/pid/mountinfo" can cause a
use-after-free so perhaps the bug lies elsewhere and was hidden by luck
(slab buffering or slab rcu-freeing or something).  In which case
"fs/seq_file: fallback to vmalloc allocation" might be the patch which
added the bug.
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