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

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Message-ID: <20080410151945.GE6725@atrey.karlin.mff.cuni.cz>
Date:	Thu, 10 Apr 2008 17:19:45 +0200
From:	Jan Kara <jack@...e.cz>
To:	Jiri Kosina <jkosina@...e.cz>
Cc:	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 Thu, 10 Apr 2008, Jan Kara wrote:
> 
> > > 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...
> 
> Supposing you are not holding any spinlock and are running with 
> preemptible kernel (pretty common scenario nowadays), there is nothing 
> that would prevent kernel from rescheduling between the two instructions, 
> enlarging the race window to be more comfortable for attacker, right?
  Yes, this is theoretically possible.

> I think this is worth fixing.
  Hmm, maybe it is, although I still don't see how to exploit it :).

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