cgroup: reject cgroup names with '\n'

/proc/<pid>/cgroup contains one cgroup path on each line. If cgroup names are
allowed to contain "\n", applications cannot parse /proc/<pid>/cgroup safely.
Signed-off-by: Alban Crequy <alban.crequy@collabora.co.uk>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org

kernel/cgroup.c

@@ -4543,6 +4543,11 @@ static int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name,
 	struct cftype *base_files;
 	int ssid, ret;
 
+	/* Do not accept '\n' to prevent making /proc/<pid>/cgroup unparsable.
+	 */
+	if (strchr(name, '\n'))
+		return -EINVAL;
+
 	parent = cgroup_kn_lock_live(parent_kn);
 	if (!parent)
 		return -ENODEV;