[PATCH] drm/i915: Fix integer overflow tests

[Jason Ekstrand]

On Thu, Aug 17, 2017 at 2:56 AM, Imre Deak <imre.deak at intel.com> wrote:

> On Thu, Aug 17, 2017 at 12:50:37PM +0300, Dan Carpenter wrote:
> > On Thu, Aug 17, 2017 at 12:37:00PM +0300, Imre Deak wrote:
> > > On Thu, Aug 17, 2017 at 09:23:10AM +0300, Dan Carpenter wrote:
> > > > There are some potential integer overflows here on 64 bit systems.
> > > >
> > > > The condition "if (nfences > SIZE_MAX / sizeof(*fences))" can only be
> > > > true on 32 bit systems, it's a no-op on 64 bit, so let's ignore the
> > > > check for now and look a couple lines after:
> > > >
> > > >   if (!access_ok(VERIFY_READ, user, nfences * 2 * sizeof(u32)))
> > > >                                           ^^^^^^^^^^^
> > > > "nfences" is an unsigned int, so if we set it to UINT_MAX and
> multiply
> > > > by two, it's going to have an integer overflow.
> > >
> > > AFAICS it wouldn't overflow due the promotion to unsigned long
> > > by '* sizeof(u32)'.
> > >
> >
> > It first multplies "nfences * 2" as unsigned int, then it type promotes
> > to size_t and multiplies by sizeof().  Only the first multiplication has
> > an integer overflow bug.
>
> Err, that's correct. Sorry for the noise.
>

Why not just replace the "2 * sizeof(u32)" with a "sizeof(*user)".  That's
what we really want to check.  I have no idea how it ended up being "2 *
sizeof(u32)"

--Jason
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