On Wed, Sep 14, 2016 at 9:00 PM, Alexei Starovoitov
[off-list ref] wrote:

On Wed, Sep 14, 2016 at 07:27:08PM -0700, Andy Lutomirski wrote:

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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