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SubjectRe: file offset corruption on 32-bit machines?
> On Thu, 10 Apr 2008, Jan Kara wrote:
> > > The f_pos races are in fact exploitable, we've already been there. See
> > > for example
> > 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?
Yes, this is theoretically possible.

> I think this is worth fixing.
Hmm, maybe it is, although I still don't see how to exploit it :).

Jan Kara <>
SuSE CR Labs

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