linux-kernel - [PATCH AUTOSEL 6.1 52/61] cgroup: Protect css->cgroup write under css_set

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Message-ID: <20240801002803.3935985-52-sashal@kernel.org>
Date: Wed, 31 Jul 2024 20:26:10 -0400
From: Sasha Levin <sashal@...nel.org>
To: linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Cc: Waiman Long <longman@...hat.com>,
	Tejun Heo <tj@...nel.org>,
	Sasha Levin <sashal@...nel.org>,
	lizefan.x@...edance.com,
	hannes@...xchg.org,
	mkoutny@...e.com,
	cgroups@...r.kernel.org
Subject: [PATCH AUTOSEL 6.1 52/61] cgroup: Protect css->cgroup write under css_set_lock

From: Waiman Long <longman@...hat.com>

[ Upstream commit 57b56d16800e8961278ecff0dc755d46c4575092 ]

The writing of css->cgroup associated with the cgroup root in
rebind_subsystems() is currently protected only by cgroup_mutex.
However, the reading of css->cgroup in both proc_cpuset_show() and
proc_cgroup_show() is protected just by css_set_lock. That makes the
readers susceptible to racing problems like data tearing or caching.
It is also a problem that can be reported by KCSAN.

This can be fixed by using READ_ONCE() and WRITE_ONCE() to access
css->cgroup. Alternatively, the writing of css->cgroup can be moved
under css_set_lock as well which is done by this patch.

Signed-off-by: Waiman Long <longman@...hat.com>
Signed-off-by: Tejun Heo <tj@...nel.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 kernel/cgroup/cgroup.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 97ecca43386d9..2656c6d8b085c 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -1843,9 +1843,9 @@ int rebind_subsystems(struct cgroup_root *dst_root, u16 ss_mask)
 		RCU_INIT_POINTER(scgrp->subsys[ssid], NULL);
 		rcu_assign_pointer(dcgrp->subsys[ssid], css);
 		ss->root = dst_root;
-		css->cgroup = dcgrp;
 
 		spin_lock_irq(&css_set_lock);
+		css->cgroup = dcgrp;
 		WARN_ON(!list_empty(&dcgrp->e_csets[ss->id]));
 		list_for_each_entry_safe(cset, cset_pos, &scgrp->e_csets[ss->id],
 					 e_cset_node[ss->id]) {
-- 
2.43.0