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

[Andy Lutomirski]

On Wed, Sep 14, 2016 at 7:19 PM, Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
> On Wed, Sep 14, 2016 at 06:25:07PM -0700, Andy Lutomirski wrote:
>> 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.
>
> Ours and Sargun's use cases for cgroup+lsm+bpf is not for security
> and not for sandboxing. So the above doesn't matter in such contexts.
> lsm hooks + cgroups provide convenient scope and existing entry points.
> Please see checmate examples how it's used.
>

To be clear: I'm not arguing at all that there shouldn't be
bpf+lsm+cgroup integration.  I'm arguing that the unprivileged
landlock interface shouldn't expose any cgroup integration, at least
until the cgroup situation settles down a lot.

--Andy