linux-kernel - Re: [PATCH mm v3 0/9] memcg: accounting for objects allocated by mkdir cgroup

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <Ypd2DW7id4M3KJJW@carbon>
Date:   Wed, 1 Jun 2022 07:22:05 -0700
From:   Roman Gushchin <roman.gushchin@...ux.dev>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Michal Koutný <mkoutny@...e.com>,
        Vasily Averin <vvs@...nvz.org>,
        Andrew Morton <akpm@...ux-foundation.org>, kernel@...nvz.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Shakeel Butt <shakeelb@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Muchun Song <songmuchun@...edance.com>, cgroups@...r.kernel.org
Subject: Re: [PATCH mm v3 0/9] memcg: accounting for objects allocated by
 mkdir cgroup

On Wed, Jun 01, 2022 at 03:05:34PM +0200, Michal Hocko wrote:
> On Wed 01-06-22 11:32:26, Michal Hocko wrote:
> > On Wed 01-06-22 11:15:43, Michal Koutny wrote:
> > > On Wed, Jun 01, 2022 at 06:43:27AM +0300, Vasily Averin <vvs@...nvz.org> wrote:
> > > > CT-901 /# cat /sys/fs/cgroup/memory/cgroup.subgroups_limit 
> > > > 512
> > > > CT-901 /# echo 3333 > /sys/fs/cgroup/memory/cgroup.subgroups_limit 
> > > > -bash: echo: write error: Operation not permitted
> > > > CT-901 /# echo 333 > /sys/fs/cgroup/memory/cgroup.subgroups_limit 
> > > > -bash: echo: write error: Operation not permitted
> > > > 
> > > > I doubt this way can be accepted in upstream, however for OpenVz
> > > > something like this it is mandatory because it much better
> > > > than nothing.
> > > 
> > > Is this customization of yours something like cgroup.max.descendants on
> > > the unified (v2) hierarchy? (Just curious.)
> > > 
> > > (It can be made inaccessible from within the subtree either with cgroup
> > > ns or good old FS permissions.)
> > 
> > So we already do have a limit to prevent somebody from running away with
> > the number of cgroups. Nice!

Yes, we do!

> > I was not aware of that and I guess this
> > looks like the right thing to do. So do we need more control and
> > accounting that this?
> 
> I have checked the actual implementation and noticed that cgroups are
> uncharged when offlined (rmdir-ed) which means that an adversary could
> still trick the limit and runaway while still consuming resources.
> 
> Roman, I guess the reason for this implementation was to avoid limit to
> trigger on setups with memcgs which can take quite some time to die?
> Would it make sense to make the implementation more strict to really act
> as gate against potential cgroups count runways?

The reasoning was that in many cases a user can't do much about dying cgroups,
so it's not clear how they should/would handle getting -EAGAIN on creating a
new cgroup (retrying will not help, obviously). Live cgroups can be easily
deleted, dying cgroups - not always.

I'm not sure about switching the semantics. I'd wait till Muchun's lru page
reparenting will be landed (could be within 1-2 releases, I guess) and then we
can check whether the whole problem is mostly gone. Honestly, I think we might
need to fix few another things, but it might be not that hard (in comparison
to what we already did).