Add FS_USERNS_FLAG to cgroup fs

allowing root in a non-init user namespace to mount it.  This should
now be safe, because

1. non-init-root cannot mount a previously unbound subsystem
2. the task doing the mount must be privileged with respect to the
   user namespace owning the cgroup namespace
3. the mounted subsystem will have its current cgroup as the root dentry.
   the permissions will be unchanged, so tasks will receive no new
   privilege over the cgroups which they did not have on the original

Signed-off-by: Serge Hallyn <>
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 173cd1e..0a403e9 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2208,12 +2208,14 @@
 	.name = "cgroup",
 	.mount = cgroup_mount,
 	.kill_sb = cgroup_kill_sb,
+	.fs_flags = FS_USERNS_MOUNT,
 static struct file_system_type cgroup2_fs_type = {
 	.name = "cgroup2",
 	.mount = cgroup_mount,
 	.kill_sb = cgroup_kill_sb,
+	.fs_flags = FS_USERNS_MOUNT,
 static int cgroup_path_ns(struct cgroup *cgrp, char *buf, size_t buflen,