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

From: Alexei Starovoitov
Date: Thu Sep 15 2016 - 00:31:42 EST

On Wed, Sep 14, 2016 at 09:08:57PM -0700, Andy Lutomirski wrote:
> On Wed, Sep 14, 2016 at 9:00 PM, Alexei Starovoitov
> <alexei.starovoitov@xxxxxxxxx> 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.

you mean the attach part of seccomp syscall that deals with no_new_priv?
sure, that's reusable.

> 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.

not sure what you mean by 'seccomp hierarchy'. The normal process
hierarchy ?
imo the main deficiency of secccomp is inability to look into arguments.
One can argue that it's a blessing, since composite args
are not yet copied into the kernel memory.
But in a lot of cases the seccomp arguments are FDs pointing
to kernel objects and if programs could examine those objects
the sandboxing scope would be more precise.
lsm+bpf solves that part and I'd still argue that it's
orthogonal to seccomp's pass/reject flow.
I mean if seccomp says 'ok' the syscall should continue executing
as normal and whatever LSM hooks were triggered by it may have
their own lsm+bpf verdicts.
Furthermore in the process hierarchy different children
should be able to set their own lsm+bpf filters that are not
related to parallel seccomp+bpf hierarchy of programs.
seccomp syscall can be an interface to attach programs
to lsm hooks, but nothing more than that.