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

[Muchun Song]

On Wed, Oct 28, 2020 at 11:50 AM 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>

Hi Andrew,

Maybe you forgot to add this to the queue for the merge window?

Thanks.

> ---
>  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
>


--
Yours,
Muchun