Patch "cgroupns: Fix the locking in copy_cgroup_ns" 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: Fix the locking in copy_cgroup_ns

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-fix-the-locking-in-copy_cgroup_ns.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 06:35:24 -0500
Subject: cgroupns: Fix the locking in copy_cgroup_ns

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

commit 7bd8830875bfa380c68f390efbad893293749324 upstream.

If "clone(CLONE_NEWCGROUP...)" is called it results in a nice lockdep valid splat.

In __cgroup_proc_write the lock ordering is:
cgroup_mutex -- through cgroup_kn_lock_live
cgroup_threadgroup_rwsem

In copy_process the guts of clone the lock ordering is:
cgroup_threadgroup_rwsem -- through threadgroup_change_begin cgroup_mutex -- through copy_namespaces -- copy_cgroup_ns

lockdep reports some a different call chains for the first ordering of cgroup_mutex and cgroup_threadgroup_rwsem but it is harder to trace. This is most definitely deadlock potential under the right circumstances.

Fix this by by skipping the cgroup_mutex and making the locking in copy_cgroup_ns mirror the locking in cgroup_post_fork which also runs during fork under the cgroup_threadgroup_rwsem.

---
 kernel/cgroup.c |    5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

@@ -6309,14 +6309,11 @@ struct cgroup_namespace *copy_cgroup_ns(
 	if (!ns_capable(user_ns, CAP_SYS_ADMIN))
 		return ERR_PTR(-EPERM);
 
-	mutex_lock(&cgroup_mutex);
+	/* It is not safe to take cgroup_mutex here */
 	spin_lock_irq(&css_set_lock);
-
 	cset = task_css_set(current);
 	get_css_set(cset);
-
 	spin_unlock_irq(&css_set_lock);
-	mutex_unlock(&cgroup_mutex);
 
 	new_ns = alloc_cgroup_ns();
 	if (IS_ERR(new_ns)) {

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