Patch "cgroup: don't online subsystems before cgroup_name/path() are operational" has been added to the 4.9-stable tree

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

cgroup: don't online subsystems before cgroup_name/path() are operational

to the 4.9-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:
cgroup-don-t-online-subsystems-before-cgroup_name-path-are-operational.patch
and it can be found in the queue-4.9 subdirectory.

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

From: Tejun Heo <[email protected]>
Date: Thu, 26 Jan 2017 16:47:28 -0500
Subject: cgroup: don't online subsystems before cgroup_name/path() are operational

From: Tejun Heo <[email protected]>

commit 07cd12945551b63ecb1a349d50a6d69d1d6feb4a upstream.

While refactoring cgroup creation, a5bca2152036 ("cgroup: factor out cgroup_create() out of cgroup_mkdir()") incorrectly onlined subsystems before the new cgroup is associated with it kernfs_node. This is fine for cgroup proper but cgroup_name/path() depend on the associated kernfs_node and if a subsystem makes the new cgroup_subsys_state visible, which they're allowed to after onlining, it can lead to NULL dereference.

The current code performs cgroup creation and subsystem onlining in cgroup_create() and cgroup_mkdir() makes the cgroup and subsystems visible afterwards. There's no reason to online the subsystems early and we can simply drop cgroup_apply_control_enable() call from cgroup_create() so that the subsystems are onlined and made visible at the same time.

---
 kernel/cgroup.c |   13 +++++--------
 1 file changed, 5 insertions(+), 8 deletions(-)

@@ -5219,6 +5219,11 @@ err_free_css:
 	return ERR_PTR(err);
 }
 
+/*
+ * The returned cgroup is fully initialized including its control mask, but
+ * it isn't associated with its kernfs_node and doesn't have the control
+ * mask applied.
+ */
 static struct cgroup *cgroup_create(struct cgroup *parent)
 {
 	struct cgroup_root *root = parent->root;

@@ -5283,11 +5288,6 @@ static struct cgroup *cgroup_create(stru
 
 	cgroup_propagate_control(cgrp);
 
-	/* @cgrp doesn't have dir yet so the following will only create csses */
-	ret = cgroup_apply_control_enable(cgrp);
-	if (ret)
-		goto out_destroy;
-
 	return cgrp;
 
 out_cancel_ref:

@@ -5295,9 +5295,6 @@ out_cancel_ref:
 out_free_cgrp:
 	kfree(cgrp);
 	return ERR_PTR(ret);
-out_destroy:
-	cgroup_destroy_locked(cgrp);
-	return ERR_PTR(ret);
 }
 
 static int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name,

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

queue-4.9/ata-sata_mv-handle-return-value-of-devm_ioremap.patch queue-4.9/libata-apply-max_sec_1024-to-all-cx1-jb-hp-devices.patch queue-4.9/libata-fix-ata-request-sense.patch queue-4.9/percpu-refcount-fix-reference-leak-during-percpu-atomic-transition.patch queue-4.9/cgroup-don-t-online-subsystems-before-cgroup_name-path-are-operational.patch