Re: [External] Re: [PATCH v5 1/7] mm: memcontrol: slab: fix obtain a reference to a freeing memcg

[Muchun Song]

On Mon, Mar 22, 2021 at 10:46 PM Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
>
> On Sat, Mar 20, 2021 at 12:38:14AM +0800, Muchun Song 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 (which is in the
> > refill_stock when cached memcg changed) to memcg.
> >
> >   rcu_read_lock()
> >   memcg = obj_cgroup_memcg(old)
> >   __memcg_kmem_uncharge(memcg)
> >       refill_stock(memcg)
> >           if (stock->cached != memcg)
> >               // css_get can change the ref counter from 0 back to 1.
> >               css_get(&memcg->css)
> >   rcu_read_unlock()
> >
> > This fix is very like the commit:
> >
> >   eefbfa7fd678 ("mm: memcg/slab: fix use after free in obj_cgroup_charge")
> >
> > Fix this by holding a reference to the memcg which is passed to the
> > __memcg_kmem_uncharge() before calling __memcg_kmem_uncharge().
> >
> > Fixes: 3de7d4f25a74 ("mm: memcg/slab: optimize objcg stock draining")
> > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
>
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
>
> Good catch! Did you trigger the WARN_ON() in
> percpu_ref_kill_and_confirm() during testing?

No. The race window is very small, it should be difficult to trigger.
When I reviewed the code here, I suddenly realized that there
might be a problem here. Very coincidental.

Thanks.