drm/i915: Reject attempted pwrites into a read-only object

If the user created a read-only object, they should not be allowed to
circumvent the write protection using the pwrite ioctl.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jon Bloomfield <jon.bloomfield@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: Jon Bloomfield <jon.bloomfield@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.william.auld@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180712185315.3288-5-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem.c

@@ -1627,6 +1627,12 @@ i915_gem_pwrite_ioctl(struct drm_device *dev, void *data,
 		goto err;
 	}
 
+	/* Writes not allowed into this read-only object */
+	if (i915_gem_object_is_readonly(obj)) {
+		ret = -EINVAL;
+		goto err;
+	}
+
 	trace_i915_gem_object_pwrite(obj, args->offset, args->size);
 
 	ret = -ENODEV;