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

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Message-Id: <200804101631.26149.mhocko@suse.cz>
Date:	Thu, 10 Apr 2008 16:31:25 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Jiri Kosina <jkosina@...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 Thursday 10 April 2008 04:01:27 pm Jiri Kosina 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? :)

Ok, but evil code needs to have access to your struct file and in such a case 
he can do worse things ;)
Or do you have some concrete (innocent looking) example? 

>
> > 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

This is different race with file position IMO. If I understand the report 
correctly, problem was with sleeping copy_to_user while the f_pos has 
changed.

Best regards
-- 
Michal Hocko
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic 
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