linux-kernel - Re: [PATCH v2] mm: memcg/slab: Fix use after free in obj_cgroup

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Message-ID: <CALvZod6uh7hV+_0FTpJehQtvw-f_UmYXNbiiu_YM_Egq4w-TUw@mail.gmail.com>
Date:   Thu, 29 Oct 2020 08:52:03 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Yafang Shao <laoar.shao@...il.com>,
        Chris Down <chris@...isdown.name>,
        Christian Brauner <christian.brauner@...ntu.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>, esyr@...hat.com,
        Suren Baghdasaryan <surenb@...gle.com>, areber@...hat.com,
        Marco Elver <elver@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Cgroups <cgroups@...r.kernel.org>, Linux MM <linux-mm@...ck.org>
Subject: Re: [PATCH v2] mm: memcg/slab: Fix use after free in obj_cgroup_charge

On Tue, Oct 27, 2020 at 8:51 PM Muchun Song <songmuchun@...edance.com> wrote:
>
> 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@...edance.com>
> Acked-by: Roman Gushchin <guro@...com>

Reviewed-by: Shakeel Butt <shakeelb@...gle.com>