drm/virtio: Ensure cached capset entries are valid before copying.

virtio_gpu_get_caps_ioctl could return success with invalid data if a
second caller to the function occurred after the entry was created in
virtio_gpu_cmd_get_capset but prior to the virtio_gpu_cmd_capset_cb
callback being called.  This could leak contents of memory as well
since the caps_cache allocation is done without zeroing.
Signed-off-by: David Riley <davidriley@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190605234423.11348-1-davidriley@chromium.orgSigned-off-by: Gerd Hoffmann <kraxel@redhat.com>

drivers/gpu/drm/virtio/virtgpu_ioctl.c

@@ -524,7 +524,6 @@ static int virtio_gpu_get_caps_ioctl(struct drm_device *dev,
 	list_for_each_entry(cache_ent, &vgdev->cap_cache, head) {
 		if (cache_ent->id == args->cap_set_id &&
 		    cache_ent->version == args->cap_set_ver) {
-			ptr = cache_ent->caps_cache;
 			spin_unlock(&vgdev->display_info_lock);
 			goto copy_exit;
 		}
@@ -535,6 +534,7 @@ static int virtio_gpu_get_caps_ioctl(struct drm_device *dev,
 	virtio_gpu_cmd_get_capset(vgdev, found_valid, args->cap_set_ver,
 				  &cache_ent);
 
+copy_exit:
 	ret = wait_event_timeout(vgdev->resp_wq,
 				 atomic_read(&cache_ent->is_valid), 5 * HZ);
 	if (!ret)
@@ -542,7 +542,6 @@ static int virtio_gpu_get_caps_ioctl(struct drm_device *dev,
 
 	ptr = cache_ent->caps_cache;
 
-copy_exit:
 	if (copy_to_user(u64_to_user_ptr(args->addr), ptr, size))
 		return -EFAULT;