drm/i915: fix integer overflow in i915_gem_do_execbuffer()

commit 44afb3a0 upstream.

On 32-bit systems, a large args->num_cliprects from userspace via ioctl
may overflow the allocation size, leading to out-of-bounds access.

This vulnerability was introduced in commit 432e58ed ("drm/i915: Avoid
allocation for execbuffer object list").
Signed-off-by: Xi Wang <xi.wang@gmail.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/gpu/drm/i915/i915_gem_execbuffer.c

@@ -1057,6 +1057,11 @@ i915_gem_do_execbuffer(struct drm_device *dev, void *data,
 			return -EINVAL;
 		}
 
+		if (args->num_cliprects > UINT_MAX / sizeof(*cliprects)) {
+			DRM_DEBUG("execbuf with %u cliprects\n",
+				  args->num_cliprects);
+			return -EINVAL;
+		}
 		cliprects = kmalloc(args->num_cliprects * sizeof(*cliprects),
 				    GFP_KERNEL);
 		if (cliprects == NULL) {