cgroup: reject cgroup names with '\n'

commit 71b1fb5c upstream.

/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>
[ kamal: backport to 3.13-stable: fixed in cgroup_create() ]
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

kernel/cgroup.c

@@ -4350,6 +4350,11 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
 	struct cgroup_subsys *ss;
 	struct super_block *sb = root->sb;
 
+	/* Do not accept '\n' to prevent making /proc/<pid>/cgroup unparsable.
+	 */
+	if (strchr(dentry->d_name.name, '\n'))
+		return -EINVAL;
+
 	/* allocate the cgroup and its ID, 0 is reserved for the root */
 	cgrp = kzalloc(sizeof(*cgrp), GFP_KERNEL);
 	if (!cgrp)