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

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Date:	Thu, 10 Apr 2008 16:01:27 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Michal Hocko <mhocko@...e.cz>
cc:	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? :)

> 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

-- 
Jiri Kosina
SUSE Labs

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