[PATCH 5.15 004/136] cgroup: Fix race condition at rebind_subsystems()

[Greg Kroah-Hartman]

From: Jing-Ting Wu <Jing-Ting.Wu@xxxxxxxxxxxx>

commit 763f4fb76e24959c370cdaa889b2492ba6175580 upstream.

Root cause:
The rebind_subsystems() is no lock held when move css object from A
list to B list,then let B's head be treated as css node at
list_for_each_entry_rcu().

Solution:
Add grace period before invalidating the removed rstat_css_node.

Reported-by: Jing-Ting Wu <jing-ting.wu@xxxxxxxxxxxx>
Suggested-by: Michal Koutný <mkoutny@xxxxxxxx>
Signed-off-by: Jing-Ting Wu <jing-ting.wu@xxxxxxxxxxxx>
Tested-by: Jing-Ting Wu <jing-ting.wu@xxxxxxxxxxxx>
Link: https://lore.kernel.org/linux-arm-kernel/d8f0bc5e2fb6ed259f9334c83279b4c011283c41.camel@xxxxxxxxxxxx/T/
Acked-by: Mukesh Ojha <quic_mojha@xxxxxxxxxxx>
Fixes: a7df69b81aac ("cgroup: rstat: support cgroup1")
Cc: stable@xxxxxxxxxxxxxxx # v5.13+
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 kernel/cgroup/cgroup.c |    1 +
 1 file changed, 1 insertion(+)

--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -1810,6 +1810,7 @@ int rebind_subsystems(struct cgroup_root
 
 		if (ss->css_rstat_flush) {
 			list_del_rcu(&css->rstat_css_node);
+			synchronize_rcu();
 			list_add_rcu(&css->rstat_css_node,
 				     &dcgrp->rstat_css_list);
 		}