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

[Andy Lutomirski <luto@amacapital.net>]

On Wed, Sep 14, 2016 at 9:00 PM, Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
> On Wed, Sep 14, 2016 at 07:27:08PM -0700, Andy Lutomirski wrote:
>> >> >
>> >> > 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.
>
> ahh. yes. we're perfectly in agreement here.
> I'm suggesting that the next RFC shouldn't include unpriv
> and seccomp at all. Once bpf+lsm+cgroup is merged, we can
> argue about unpriv with cgroups and even unpriv as a whole,
> since it's not a given. Seccomp integration is also questionable.
> I'd rather not have seccomp as a gate keeper for this lsm.
> lsm and seccomp are orthogonal hook points. Syscalls and lsm hooks
> don't have one to one relationship, so mixing them up is only
> asking for trouble further down the road.
> If we really need to carry some information from seccomp to lsm+bpf,
> it's easier to add eBPF support to seccomp and let bpf side deal
> with passing whatever information.
>

As an argument for keeping seccomp (or an extended seccomp) as the
interface for an unprivileged bpf+lsm: seccomp already checks off most
of the boxes for safely letting unprivileged programs sandbox
themselves.  Furthermore, to the extent that there are use cases for
unprivileged bpf+lsm that *aren't* expressible within the seccomp
hierarchy, I suspect that syscall filters have exactly the same
problem and that we should fix seccomp to cover it.

If I ever add a "seccomp monitor", which is something I want to do
eventually, I think it should work for lsm+bpf as well, which is
another argument for keeping it in seccomp.

--Andy