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Re: [PATCH] vfprintf stack overflow [BZ #16617]

On Fri, Dec 05, 2014 at 05:01:32PM +0000, Joseph Myers wrote:
> On Fri, 5 Dec 2014, Florian Weimer wrote:
> > This fell through the cracks.  I took Jeff Law's patch (which we carry as a
> > local patch in Fedora and downstream), compressed the bug23-3.c test case, and
> > added Joseph's test case from the bug as bug23-4.c.
> What's your view of the other possible overflows there that Paul Eggert 
> mentioned in <>?  
> I think nspecs * sizeof (*specs) is always OK (that's the size of an 
> object that's already been allocated), but 2 * nspecs_size might not be 
> (if it can't overflow in practice, that's an accident to do with the 
> size of struct printf_spec, the particular sequence of allocation sizes 
> and how much memory it's actually possible to allocate on existing 
> systems, rather than because the code is sensible to keep as-is without a 
> check on that multiplication).

If N is the size of an actual allocated object, 2*N should not be able
to overflow. If it can, it means you already have a situation where an
object is so large that legal pointer subtractions overflow ptrdiff_t,
which is a dangerous situation to begin with and should be fixed.


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