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

[Shakeel Butt]

On Tue, Oct 27, 2020 at 8:51 PM Muchun Song <songmuchun@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> Acked-by: Roman Gushchin <guro@xxxxxx>

Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>