linux-kernel - [PATCH 2/2] cgroup: make sure a parent css isn't freed before its children

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Date:	Thu, 21 Jan 2016 15:32:15 -0500
From:	Tejun Heo <tj@...nel.org>
To:	Christian Borntraeger <borntraeger@...ibm.com>
Cc:	linux-kernel@...r.kernel.org,
	linux-s390 <linux-s390@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Li Zefan <lizefan@...wei.com>,
	Johannes Weiner <hannes@...xchg.org>, cgroups@...r.kernel.org,
	kernel-team@...com
Subject: [PATCH 2/2] cgroup: make sure a parent css isn't freed before its
 children

There are three subsystem callbacks in css shutdown path -
css_offline(), css_released() and css_free().  Except for
css_released(), cgroup core didn't use to guarantee the order of
invocation.  css_offline() or css_free() could be called on a parent
css before its children.  This behavior is unexpected and led to
use-after-free in cpu controller.

The previous patch updated ordering for css_offline() which fixes the
cpu controller issue.  While there currently isn't a known bug caused
by misordering of css_free() invocations, let's fix it too for
consistency.

css_free() ordering can be trivially fixed by moving putting of the
parent css below css_free() invocation.

Signed-off-by: Tejun Heo <tj@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
---
 kernel/cgroup.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4657,14 +4657,15 @@ static void css_free_work_fn(struct work

 	if (ss) {
 		/* css free path */
+		struct cgroup_subsys_state *parent = css->parent;
 		int id = css->id;

-		if (css->parent)
-			css_put(css->parent);
-
 		ss->css_free(css);
 		cgroup_idr_remove(&ss->css_idr, id);
 		cgroup_put(cgrp);
+
+		if (parent)
+			css_put(parent);
 	} else {
 		/* cgroup free path */
 		atomic_dec(&cgrp->root->nr_cgrps);