[PATCH v2] mm: memcg/slab: Fix use after free in obj_cgroup_charge

From: Muchun Song <songmuchun@bytedance.com>
To: hannes@cmpxchg.org, mhocko@kernel.org, vdavydov.dev@gmail.com,
	akpm@linux-foundation.org, shakeelb@google.com, guro@fb.com,
	iamjoonsoo.kim@lge.com, laoar.shao@gmail.com,
	chris@chrisdown.name, christian.brauner@ubuntu.com,
	peterz@infradead.org, mingo@kernel.org, keescook@chromium.org,
	tglx@linutronix.de, esyr@redhat.com, surenb@google.com,
	areber@redhat.com, elver@google.com
Cc: linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
	linux-mm@kvack.org, Muchun Song <songmuchun@bytedance.com>
Subject: [PATCH v2] mm: memcg/slab: Fix use after free in obj_cgroup_charge
Date: Wed, 28 Oct 2020 11:50:11 +0800	[thread overview]
Message-ID: <20201028035013.99711-2-songmuchun@bytedance.com> (raw)
In-Reply-To: <20201028035013.99711-1-songmuchun@bytedance.com>

The rcu_read_lock/unlock only can guarantee that the memcg will
not be freed, but it cannot guarantee the success of css_get to
memcg.

If the whole process of a cgroup offlining is completed between
reading a objcg->memcg pointer and bumping the css reference on
another CPU, and there are exactly 0 external references to this
memory cgroup (how we get to the obj_cgroup_charge() then?),
css_get() can change the ref counter from 0 back to 1.

Fixes: bf4f059954dc ("mm: memcg/slab: obj_cgroup API")
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Roman Gushchin <guro@fb.com>
---
 changelog in v2:
 1. Add unlikely and update the commit log suggested by Roman.

 mm/memcontrol.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 8c8b4c3ed5a0..d9cdf899c6fc 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3221,8 +3221,10 @@ int obj_cgroup_charge(struct obj_cgroup *objcg, gfp_t gfp, size_t size)
 	 * independently later.
 	 */
 	rcu_read_lock();
+retry:
 	memcg = obj_cgroup_memcg(objcg);
-	css_get(&memcg->css);
+	if (unlikely(!css_tryget(&memcg->css)))
+		goto retry;
 	rcu_read_unlock();
 
 	nr_pages = size >> PAGE_SHIFT;
-- 
2.20.1

