cgroup: reject cgroup names with ' '

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>
[lizf: Backported to 3.4:
 - adjust context
 - s/name/dentry->d_name.name/]
Signed-off-by: Zefan Li <lizefan@huawei.com>

kernel/cgroup.c

@@ -3838,6 +3838,11 @@ static int cgroup_mkdir(struct inode *dir, struct dentry *dentry, umode_t mode)
 {
 	struct cgroup *c_parent = dentry->d_parent->d_fsdata;
 
+	/* Do not accept '\n' to prevent making /proc/<pid>/cgroup unparsable.
+	 */
+	if (strchr(dentry->d_name.name, '\n'))
+		return -EINVAL;
+
 	/* the vfs holds inode->i_mutex already */
 	return cgroup_create(c_parent, dentry, mode | S_IFDIR);
 }