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

[Tianyin Xu <tyxu@illinois.edu>]

On Thu, May 20, 2021 at 3:56 AM Christian Brauner
<christian.brauner@ubuntu.com> wrote:
>
> On Thu, May 20, 2021 at 03:16:10AM -0500, Tianyin Xu wrote:
> > On Mon, May 17, 2021 at 10:40 AM Tycho Andersen <tycho@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@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@kernel.org> wrote:
> > > > > >> On Mon, May 10, 2021 at 10:22 AM YiFei Zhu <zhuyifei1999@gmail.com> wrote:
> > > > > >>>
> > > > > >>> From: YiFei Zhu <yifeifz2@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
>
> That seems like a strawman. I don't see why this has to be out of
> process or a separate daemon. Conmon uses a regular event loop. Adding
> support for processing notifier syscall notifications is
> straightforward. Moving it to a plugin as you mentioned below is a
> design decision not a necessity.
>
> >
> > Apparently, nobody likes one more daemon. So, the proposal from
>
> I'm not sure such a blanket statements about an indeterminate group of
> people's alleged preferences constitutes a technical argument wny we
> need ebpf in seccomp.
>
> > Giuseppe was/is to use user notifiers as plugins (.so) loaded by
> > conmon:
> > https://urldefense.com/v3/__https://github.com/containers/conmon/pull/190__;!!DZ3fjg!qFZ7PXfFe7eI1Bye9J8zsGOxTQQlfL-pBh0D7Arn1YZKevtEpA9uxKqMTP9kA5RJ$
> > https://urldefense.com/v3/__https://github.com/containers/crun/pull/438__;!!DZ3fjg!qFZ7PXfFe7eI1Bye9J8zsGOxTQQlfL-pBh0D7Arn1YZKevtEpA9uxKqMTJrKzhUD$
> >
> > 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.
>
> So I think this is trying to jump the gun by saying "Look, the result
> might be simpler.". That may even be the case - though I'm not yet
> convinced - but Andy's point stands that this brings a slew of issues on
> the table that need clear answers. Bringing stateful ebpf features into
> seccomp is a pretty big step and especially around the
> privilege/security model it looks pretty handwavy right now.
>
> Christian

If an alleged gunshot was the impression I left, I apologize.
Seriously, I have great respect for user notifiers -- my intention was
never to disregard it, or to argue that ebpf filters are always
strictly better.

On the other hand, I do believe (and tried to show) ebpf filters have
their own technical advantages, and can be very useful and efficient
in many use cases. Let me know if you don’t buy this.

It’s kinda weird that we are arguing against ebpf filters with user
notifiers (it’s analogous to "we don’t need Seccomp coz we have
ptrace…")

More importantly, I do really want to provide clear answers to the
privilege/security model, but I don’t precisely know what exactly
you’re referring to as "privilege/security model". Are you referring
to the one-way transition model of Seccomp (which may no longer be
held in stateful filters), or something different? It will be great if
you can clarify so we can answer explicitly.

Thanks!