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Tue, 27 Oct 2020 20:50:56 -0700 (PDT) From: Muchun Song 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 Subject: [PATCH v2] mm: memcg/slab: Fix use after free in obj_cgroup_charge Date: Wed, 28 Oct 2020 11:50:11 +0800 Message-Id: <20201028035013.99711-2-songmuchun@bytedance.com> X-Mailer: git-send-email 2.21.0 (Apple Git-122) In-Reply-To: <20201028035013.99711-1-songmuchun@bytedance.com> References: <20201028035013.99711-1-songmuchun@bytedance.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000032, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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 Acked-by: Roman Gushchin --- 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, gf= p_t gfp, size_t size) * independently later. */ rcu_read_lock(); +retry: memcg =3D obj_cgroup_memcg(objcg); - css_get(&memcg->css); + if (unlikely(!css_tryget(&memcg->css))) + goto retry; rcu_read_unlock(); =20 nr_pages =3D size >> PAGE_SHIFT; --=20 2.20.1