Patch "cgroupns: Only allow creation of hierarchies in the initial cgroup namespace" has been added to the 4.7-stable tree

This is a note to let you know that I've just added the patch titled

cgroupns: Only allow creation of hierarchies in the initial cgroup namespace

to the 4.7-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
cgroupns-only-allow-creation-of-hierarchies-in-the-initial-cgroup-namespace.patch
and it can be found in the queue-4.7 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: "Eric W. Biederman" <[email protected]>
Date: Fri, 15 Jul 2016 06:36:44 -0500
Subject: cgroupns: Only allow creation of hierarchies in the initial cgroup namespace

From: Eric W. Biederman <[email protected]>

commit 726a4994b05ff5b6f83d64b5b43c3251217366ce upstream.

Unprivileged users can't use hierarchies if they create them as they do not have privilieges to the root directory.

Which means the only thing a hiearchy created by an unprivileged user is good for is expanding the number of cgroup links in every css_set, which is a DOS attack.

We could allow hierarchies to be created in namespaces in the initial user namespace. Unfortunately there is only a single namespace for the names of heirarchies, so that is likely to create more confusion than not.

So do the simple thing and restrict hiearchy creation to the initial cgroup namespace.

---
 kernel/cgroup.c |    8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

@@ -2215,12 +2215,8 @@ static struct dentry *cgroup_mount(struc
 		goto out_unlock;
 	}
 
-	/*
-	 * We know this subsystem has not yet been bound.  Users in a non-init
-	 * user namespace may only mount hierarchies with no bound subsystems,
-	 * i.e. 'none,name=user1'
-	 */
-	if (!opts.none && !capable(CAP_SYS_ADMIN)) {
+	/* Hierarchies may only be created in the initial cgroup namespace. */
+	if (ns != &init_cgroup_ns) {
 		ret = -EPERM;
 		goto out_unlock;
 	}

Patches currently in stable-queue which might be from [email protected] are

queue-4.7/cgroupns-fix-the-locking-in-copy_cgroup_ns.patch queue-4.7/cgroupns-only-allow-creation-of-hierarchies-in-the-initial-cgroup-namespace.patch queue-4.7/cgroupns-close-race-between-cgroup_post_fork-and-copy_cgroup_ns.patch