drm/i915/userptr: add user_size limit check

Don't allow a mismatch between obj->base.size/vma->size and the actual
number of pages for the backing store, which is limited to INT_MAX
pages.

v2: document what are missing before we can safely drop the limit check
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200117132413.1170563-1-matthew.auld@intel.com

drivers/gpu/drm/i915/gem/i915_gem_userptr.c

@@ -769,6 +769,23 @@ i915_gem_userptr_ioctl(struct drm_device *dev,
 			    I915_USERPTR_UNSYNCHRONIZED))
 		return -EINVAL;
 
+	/*
+	 * XXX: There is a prevalence of the assumption that we fit the
+	 * object's page count inside a 32bit _signed_ variable. Let's document
+	 * this and catch if we ever need to fix it. In the meantime, if you do
+	 * spot such a local variable, please consider fixing!
+	 *
+	 * Aside from our own locals (for which we have no excuse!):
+	 * - sg_table embeds unsigned int for num_pages
+	 * - get_user_pages*() mixed ints with longs
+	 */
+
+	if (args->user_size >> PAGE_SHIFT > INT_MAX)
+		return -E2BIG;
+
+	if (overflows_type(args->user_size, obj->base.size))
+		return -E2BIG;
+
 	if (!args->user_size)
 		return -EINVAL;