Re: [RFC v3 18/22] cgroup,landlock: Add CGRP_NO_NEW_PRIVS to handle unprivileged hooks

[Andy Lutomirski]

On Wed, Sep 14, 2016 at 3:11 PM, MickaÃl SalaÃn <mic@xxxxxxxxxxx> wrote:
>
> On 14/09/2016 20:27, Andy Lutomirski wrote:
>> On Wed, Sep 14, 2016 at 12:24 AM, MickaÃl SalaÃn <mic@xxxxxxxxxxx> wrote:
>>> Add a new flag CGRP_NO_NEW_PRIVS for each cgroup. This flag is initially
>>> set for all cgroup except the root. The flag is clear when a new process
>>> without the no_new_privs flags is attached to the cgroup.
>>>
>>> If a cgroup is landlocked, then any new attempt, from an unprivileged
>>> process, to attach a process without no_new_privs to this cgroup will
>>> be denied.
>>
>> Until and unless everyone can agree on a way to properly namespace,
>> delegate, etc cgroups, I think that trying to add unprivileged
>> semantics to cgroups is nuts.  Given the big thread about cgroup v2,
>> no-internal-tasks, etc, I just don't see how this approach can be
>> viable.
>
> As far as I can tell, the no_new_privs flag of at task is not related to
> namespaces. The CGRP_NO_NEW_PRIVS flag is only a cache to quickly access
> the no_new_privs property of *tasks* in a cgroup. The semantic is unchanged.
>
> Using cgroup is optional, any task could use the seccomp-based
> landlocking instead. However, for those that want/need to manage a
> security policy in a more dynamic way, using cgroups may make sense.
>
> I though cgroup delegation was OK in the v2, isn't it the case? Do you
> have some links?
>
>>
>> Can we try to make landlock work completely independently of cgroups
>> so that it doesn't get stuck and so that programs can use it without
>> worrying about cgroup v1 vs v2, interactions with cgroup managers,
>> cgroup managers that (supposedly?) will start migrating processes
>> around piecemeal and almost certainly blowing up landlock in the
>> process, etc?
>
> This RFC handle both cgroup and seccomp approaches in a similar way. I
> don't see why building on top of cgroup v2 is a problem. Is there
> security issues with delegation?

What I mean is: cgroup v2 delegation has a functionality problem.
Tejun says [1]:

We haven't had to face this decision because cgroup has never properly
supported delegating to applications and the in-use setups where this
happens are custom configurations where there is no boundary between
system and applications and adhoc trial-and-error is good enough a way
to find a working solution.  That wiggle room goes away once we
officially open this up to individual applications.

Unless and until that changes, I think that landlock should stay away
from cgroups.  Others could reasonably disagree with me.


[1] https://lkml.kernel.org/g/<20160909225747.GA30105@xxxxxxxxxxxxxxx