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

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Message-ID: <20080410144839.GQ11962@parisc-linux.org>
Date:	Thu, 10 Apr 2008 08:48:39 -0600
From:	Matthew Wilcox <matthew@....cx>
To:	Jiri Kosina <jkosina@...e.cz>
Cc:	Jan Kara <jack@...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?

-- 
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