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

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Message-ID: <20080410152212.GF6725@atrey.karlin.mff.cuni.cz>
Date:	Thu, 10 Apr 2008 17:22:12 +0200
From:	Jan Kara <jack@...e.cz>
To:	Matthew Wilcox <matthew@....cx>
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 Thu, Apr 10, 2008 at 04:31:09PM +0200, Jiri Kosina wrote:
> > >   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?
> > 
> > I think this is worth fixing.
> 
> Seems a lot like reading jiffies to me.  Is the seqlock the right
> solution to use for fixing this?
  You can get your inspiration in the implementation of i_size_read()
and i_size_write() functions :). They deal with exactly the same problem.
But in the case of f_pos, the number of readers and writers is balanced so
maybe a spinlock would be fine as well...

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