virtiofs: forbid newlines in tags

Newlines in virtiofs tags are awkward for users and potential vectors
for string injection attacks.

Signed-off-by: Stefan Hajnoczi <[e-mail]>
Reviewed-by: Vivek Goyal <[e-mail]>
Signed-off-by: Miklos Szeredi <[e-mail]>

fs/fuse/virtio_fs.c

···
 		return -ENOMEM;
 	memcpy(fs->tag, tag_buf, len);
 	fs->tag[len] = '\0';
+
+	/* While the VIRTIO specification allows any character, newlines are
+	 * awkward on mount(8) command-lines and cause problems in the sysfs
+	 * "tag" attr and uevent TAG= properties. Forbid them.
+	 */
+	if (strchr(fs->tag, '\n')) {
+		dev_dbg(&vdev->dev, "refusing virtiofs tag with newline character\n");
+		return -EINVAL;
+	}
+
 	return 0;
 }