[RFC PATCH bpf-next seccomp 00/12] eBPF seccomp filters

[Tianyin Xu]

On Mon, May 17, 2021 at 10:40 AM Tycho Andersen <tycho at tycho.pizza> wrote:
>
> On Sun, May 16, 2021 at 03:38:00AM -0500, Tianyin Xu wrote:
> > On Sat, May 15, 2021 at 10:49 AM Andy Lutomirski <luto at kernel.org> wrote:
> > >
> > > On 5/10/21 10:21 PM, YiFei Zhu wrote:
> > > > On Mon, May 10, 2021 at 12:47 PM Andy Lutomirski <luto at kernel.org> wrote:
> > > >> On Mon, May 10, 2021 at 10:22 AM YiFei Zhu <zhuyifei1999 at gmail.com> wrote:
> > > >>>
> > > >>> From: YiFei Zhu <yifeifz2 at illinois.edu>
> > > >>>
> > > >>> Based on: https://urldefense.com/v3/__https://lists.linux-foundation.org/pipermail/containers/2018-February/038571.html__;!!DZ3fjg!thbAoRgmCeWjlv0qPDndNZW1j6Y2Kl_huVyUffr4wVbISf-aUiULaWHwkKJrNJyo$
> > > >>>
> > > >>> This patchset enables seccomp filters to be written in eBPF.
> > > >>> Supporting eBPF filters has been proposed a few times in the past.
> > > >>> The main concerns were (1) use cases and (2) security. We have
> > > >>> identified many use cases that can benefit from advanced eBPF
> > > >>> filters, such as:
> > > >>
> > > >> I haven't reviewed this carefully, but I think we need to distinguish
> > > >> a few things:
> > > >>
> > > >> 1. Using the eBPF *language*.
> > > >>
> > > >> 2. Allowing the use of stateful / non-pure eBPF features.
> > > >>
> > > >> 3. Allowing the eBPF programs to read the target process' memory.
> > > >>
> > > >> I'm generally in favor of (1).  I'm not at all sure about (2), and I'm
> > > >> even less convinced by (3).
> > > >>
> > > >>>
> > > >>>   * exec-only-once filter / apply filter after exec
> > > >>
> > > >> This is (2).  I'm not sure it's a good idea.
> > > >
> > > > The basic idea is that for a container runtime it may wait to execute
> > > > a program in a container without that program being able to execve
> > > > another program, stopping any attack that involves loading another
> > > > binary. The container runtime can block any syscall but execve in the
> > > > exec-ed process by using only cBPF.
> > > >
> > > > The use case is suggested by Andrea Arcangeli and Giuseppe Scrivano.
> > > > @Andrea and @Giuseppe, could you clarify more in case I missed
> > > > something?
> > >
> > > We've discussed having a notifier-using filter be able to replace its
> > > filter.  This would allow this and other use cases without any
> > > additional eBPF or cBPF code.
> > >
> >
> > A notifier is not always a solution (even ignoring its perf overhead).
> >
> > One problem, pointed out by Andrea Arcangeli, is that notifiers need
> > userspace daemons. So, it can hardly be used by daemonless container
> > engines like Podman.
>
> I'm not sure I buy this argument. Podman already has a conmon instance
> for each container, this could be a child of that conmon process, or
> live inside conmon itself.
>
> Tycho

I checked with Andrea Arcangeli and Giuseppe Scrivano who are working on Podman.

You are right that Podman is not completely daemonless. However, “the
fact it's no entirely daemonless doesn't imply it's a good idea to
make it worse and to add complexity to the background conmon daemon or
to add more daemons.”

TL;DR. User notifiers are surely more flexible, but are also more
expensive and complex to implement, compared with ebpf filters. /*
I’ll reply to Sargun’s performance argument in a separate email */

I'm sure you know Podman well, but let me still move some jade from
Andrea and Giuseppe (all credits on podmon/crun are theirs) to
elaborate the point, for folks cced on the list who are not very
familiar with Podman.

Basically, the current order goes as follows:

         podman -> conmon -> crun -> container_binary
                               \
                                - seccomp done at crun level, not conmon

At runtime, what's left is:

         conmon -> container_binary  /* podman disappears; crun disappears */

So, to go through and use seccomp notify to block `exec`, we can
either start the container_binary with a seccomp agent wrapper, or
bloat the common binary (as pointed out by Tycho).

If we go with the first approach, we will have:

         podman -> conmon -> crun -> seccomp_agent -> container_binary

So, at runtime we'd be left with one more daemon:

        conmon -> seccomp_agent -> container_binary

Apparently, nobody likes one more daemon. So, the proposal from
Giuseppe was/is to use user notifiers as plugins (.so) loaded by
conmon:
https://github.com/containers/conmon/pull/190
https://github.com/containers/crun/pull/438

Now, with the ebpf filter support, one can implement the same thing
using an embarrassingly simple ebpf filter and, thanks to Giuseppe,
this is well supported by crun.

-- 
Tianyin Xu
University of Illinois at Urbana-Champaign
https://tianyin.github.io/