[Intel-gfx] drm/i915: Possible security hole in command parsing

[Rebecca N. Palmer]

In next, it was fixed by 
http://cgit.freedesktop.org/drm-intel/commit/drivers/gpu/drm/i915/i915_gem_execbuffer.c?h=for-linux-next&id=c7c7372edc4ebc173ad359aeb5752e9ce09f2741

In 4.1, the problem only exists with the marked-as-unsafe parameter 
i915.enable_ppgtt=2.  (It could be fixed there with 
http://lists.freedesktop.org/archives/intel-gfx/2015-May/066259.html , 
but doing so at this late stage would be risky.)