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    Subject[PATCH 5.10 450/717] mm: memcg/slab: fix use after free in obj_cgroup_charge
    Date
    From: Muchun Song <songmuchun@bytedance.com>

    [ Upstream commit eefbfa7fd678805b38a46293e78543f98f353d3e ]

    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.

    Link: https://lkml.kernel.org/r/20201028035013.99711-2-songmuchun@bytedance.com
    Fixes: bf4f059954dc ("mm: memcg/slab: obj_cgroup API")
    Signed-off-by: Muchun Song <songmuchun@bytedance.com>
    Acked-by: Roman Gushchin <guro@fb.com>
    Reviewed-by: Shakeel Butt <shakeelb@google.com>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Michal Hocko <mhocko@kernel.org>
    Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
    Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
    Cc: Yafang Shao <laoar.shao@gmail.com>
    Cc: Chris Down <chris@chrisdown.name>
    Cc: Christian Brauner <christian.brauner@ubuntu.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    mm/memcontrol.c | 4 +++-
    1 file changed, 3 insertions(+), 1 deletion(-)

    diff --git a/mm/memcontrol.c b/mm/memcontrol.c
    index 74b85077f89ad..a717728cc7b4a 100644
    --- a/mm/memcontrol.c
    +++ b/mm/memcontrol.c
    @@ -3247,8 +3247,10 @@ 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;
    --
    2.27.0


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