Linux Ethernet Bridging - Dec 2018 - [Bridge] [PATCH] netfilter: account ebt_table_info to kmemcg

On Sat, Dec 29, 2018 at 2:06 AM Michal Hocko <mhocko at kernel.org>
wrote:
>
> On Sat 29-12-18 10:52:15, Florian Westphal wrote:
> > Michal Hocko <mhocko at kernel.org> wrote:
> > > On Fri 28-12-18 17:55:24, Shakeel Butt wrote:
> > > > The [ip,ip6,arp]_tables use x_tables_info internally and the
underlying
> > > > memory is already accounted to kmemcg. Do the same for
ebtables. The
> > > > syzbot, by using setsockopt(EBT_SO_SET_ENTRIES), was able to
OOM the
> > > > whole system from a restricted memcg, a potential DoS.
> > >
> > > What is the lifetime of these objects? Are they bound to any
process?
> >
> > No, they are not.
> > They are free'd only when userspace requests it or the netns is
> > destroyed.
>
> Then this is problematic, because the oom killer is not able to
> guarantee the hard limit and so the excessive memory consumption cannot
> be really contained. As a result the memcg will be basically useless
> until somebody tears down the charged objects by other means. The memcg
> oom killer will surely kill all the existing tasks in the cgroup and
> this could somehow reduce the problem. Maybe this is sufficient for
> some usecases but that should be properly analyzed and described in the
> changelog.
>
Can you explain why you think the memcg hard limit will not be
enforced? From what I understand, the memcg oom-killer will kill the
allocating processes as you have mentioned. We do force charging for
very limited conditions but here the memcg oom-killer will take care
of

Anyways, the kernel is already charging the memory for
[ip,ip6,arp]_tables and this patch adds the charging for ebtables.
Without this patch, as Kirill has described and shown by syzbot, a low
priority memcg can force system OOM.

Shakeel

On Sat 29-12-18 11:34:29, Shakeel Butt wrote:
> On Sat, Dec 29, 2018 at 2:06 AM Michal Hocko <mhocko at kernel.org>
wrote:
> >
> > On Sat 29-12-18 10:52:15, Florian Westphal wrote:
> > > Michal Hocko <mhocko at kernel.org> wrote:
> > > > On Fri 28-12-18 17:55:24, Shakeel Butt wrote:
> > > > > The [ip,ip6,arp]_tables use x_tables_info internally
and the underlying
> > > > > memory is already accounted to kmemcg. Do the same for
ebtables. The
> > > > > syzbot, by using setsockopt(EBT_SO_SET_ENTRIES), was
able to OOM the
> > > > > whole system from a restricted memcg, a potential DoS.
> > > >
> > > > What is the lifetime of these objects? Are they bound to any
process?
> > >
> > > No, they are not.
> > > They are free'd only when userspace requests it or the netns
is
> > > destroyed.
> >
> > Then this is problematic, because the oom killer is not able to
> > guarantee the hard limit and so the excessive memory consumption
cannot
> > be really contained. As a result the memcg will be basically useless
> > until somebody tears down the charged objects by other means. The
memcg
> > oom killer will surely kill all the existing tasks in the cgroup and
> > this could somehow reduce the problem. Maybe this is sufficient for
> > some usecases but that should be properly analyzed and described in
the
> > changelog.
> >
> 
> Can you explain why you think the memcg hard limit will not be
> enforced? From what I understand, the memcg oom-killer will kill the
> allocating processes as you have mentioned. We do force charging for
> very limited conditions but here the memcg oom-killer will take care
> of
I was talking about the force charge part. Depending on a specific
allocation and its life time this can gradually get us over hard limit
without any bound theoretically.
> Anyways, the kernel is already charging the memory for
> [ip,ip6,arp]_tables and this patch adds the charging for ebtables.
> Without this patch, as Kirill has described and shown by syzbot, a low
> priority memcg can force system OOM.
I am not opposing the patch per-se. I would just like the changelog to
be more descriptive about the life time and consequences.
-- 
Michal Hocko
SUSE Labs