linux-kernel - Re: file offset corruption on 32-bit machines?

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Message-ID: <20080414162546.GE15824@duck.suse.cz>
Date:	Mon, 14 Apr 2008 18:25:46 +0200
From:	Jan Kara <jack@...e.cz>
To:	Pavel Machek <pavel@....cz>
Cc:	Jiri Kosina <jkosina@...e.cz>, Michal Hocko <mhocko@...e.cz>,
	Meelis Roos <mroos@...ux.ee>,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org
Subject: Re: file offset corruption on 32-bit machines?

On Fri 11-04-08 21:26:56, Pavel Machek wrote:
> On Thu 2008-04-10 16:27:00, Jan Kara wrote:
> > > On Thu, 10 Apr 2008, Michal Hocko wrote:
> > > 
> > > > > Jeff Robertson analyzes the behaviour of different operating systems'
> > > > > 64-bit file offset implementation and concludes that on 32-bit
> > > > > machines, Linux and Solaris lack any locking to keep the two 32-bit
> > > > > halves in sync and this could cause rare file offset corruption.
> > > > > http://jeffr-tech.livejournal.com/21014.html
> > > > AFAICS, this race is theoretically possible, but it is very hard (almost 
> > > > impossible) to trigger with a sane file usage pattern. Note that you 
> > > > have to access shared struct file (same file descriptor) in different 
> > > > threads which should be synchronized by caller anyway (*).
> > > 
> > > ... but not in cases the caller is an intentionally evil code, right? :)
> >   Yes.
> > 
> > > > I also don't see any security implications from this race, but maybe 
> > > > someone with more knowlage about fs can see (f_pos is used at many 
> > > > places in the kernel code).
> > > 
> > > The f_pos races are in fact exploitable, we've already been there. See 
> > > for example http://www.isec.pl/vulnerabilities/isec-0016-procleaks.txt
> >   Well, this race is more subtle - the window is just one instruction
> > wide (stores to f_pos from CPU2 must come between the store of lower and
> > upper 32-bits of f_pos on CPU1). And the only result is that f_pos has
> > 32-bits from one file pointer and 32-bits from the other one. So I can
> > hardly imagine this would be exploitable...
> 
> Don't we have rlimit on max file size? I'd guess this could work
> around it?
  There is this limit but AFAIK it limits max size of file you're able to
create. And write/truncate checks already their local variable so the real
value used later.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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