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

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Message-ID: <20080410142700.GC6725@atrey.karlin.mff.cuni.cz>
Date:	Thu, 10 Apr 2008 16:27:00 +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, 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...

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