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

[Sargun Dhillon <sargun@sargun.me>]

On Sun, May 16, 2021 at 1:39 AM Tianyin Xu <tyxu@illinois.edu> 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.
>
> And, /* sorry for repeating.. */ the performance overhead of notifiers
> is not close to ebpf, which prevents use cases that require native
> performance.

While I agree with you that this is the case right now, there's no reason it
has to be the case. There's a variety of mechanisms that can be employed
to significantly speed up the performance of the notifier. For example, right
now the notifier is behind one large per-filter lock. That could be removed
allowing for better concurrency. There are a large number of mechanisms
that scale O(n) with the outstanding notifications -- again, something
that could be improved.

The other big improvement that could be made is being able to use something
like io_uring with the notifier interface, but it would require a
fairly significant
user API change -- and a move away from ioctl. I'm not sure if people are
excited about that idea at the moment.

>
>
> > >> eBPF doesn't really have a privilege model yet.  There was a long and
> > >> disappointing thread about this awhile back.
> > >
> > > The idea is that “seccomp-eBPF does not make life easier for an
> > > adversary”. Any attack an adversary could potentially utilize
> > > seccomp-eBPF, they can do the same with other eBPF features, i.e. it
> > > would be an issue with eBPF in general rather than specifically
> > > seccomp’s use of eBPF.
> > >
> > > Here it is referring to the helpers goes to the base
> > > bpf_base_func_proto if the caller is unprivileged (!bpf_capable ||
> > > !perfmon_capable). In this case, if the adversary would utilize eBPF
> > > helpers to perform an attack, they could do it via another
> > > unprivileged prog type.
> > >
> > > That said, there are a few additional helpers this patchset is adding:
> > > * get_current_uid_gid
> > > * get_current_pid_tgid
> > >   These two provide public information (are namespaces a concern?). I
> > > have no idea what kind of exploit it could add unless the adversary
> > > somehow side-channels the task_struct? But in that case, how is the
> > > reading of task_struct different from how the rest of the kernel is
> > > reading task_struct?
> >
> > Yes, namespaces are a concern.  This idea got mostly shot down for kdbus
> > (what ever happened to that?), and it likely has the same problems for
> > seccomp.
> >
> > >>
> > >> What is this for?
> > >
> > > Memory reading opens up lots of use cases. For example, logging what
> > > files are being opened without imposing too much performance penalty
> > > from strace. Or as an accelerator for user notify emulation, where
> > > syscalls can be rejected on a fast path if we know the memory contents
> > > does not satisfy certain conditions that user notify will check.
> > >
> >
> > This has all kinds of race conditions.
> >
> >
> > I hate to be a party pooper, but this patchset is going to very high bar
> > to acceptance.  Right now, seccomp has a couple of excellent properties:
> >
> > First, while it has limited expressiveness, it is simple enough that the
> > implementation can be easily understood and the scope for
> > vulnerabilities that fall through the cracks of the seccomp sandbox
> > model is low.  Compare this to Windows' low-integrity/high-integrity
> > sandbox system: there is a never ending string of sandbox escapes due to
> > token misuse, unexpected things at various integrity levels, etc.
> > Seccomp doesn't have tokens or integrity levels, and these bugs don't
> > happen.
> >
> > Second, seccomp works, almost unchanged, in a completely unprivileged
> > context.  The last time making eBPF work sensibly in a less- or
> > -unprivileged context, the maintainers mostly rejected the idea of
> > developing/debugging a permission model for maps, cleaning up the bpf
> > object id system, etc.  You are going to have a very hard time
> > convincing the seccomp maintainers to let any of these mechanism
> > interact with seccomp until the underlying permission model is in place.
> >
> > --Andy
>
> Thanks for pointing out the tradeoff between expressiveness vs. simplicity.
>
> Note that we are _not_ proposing to replace cbpf, but propose to also
> support ebpf filters. There certainly are use cases where cbpf is
> sufficient, but there are also important use cases ebpf could make
> life much easier.
>
> Most importantly, we strongly believe that ebpf filters can be
> supported without reducing security.
>
> No worries about “party pooping” and we appreciate the feedback. We’d
> love to hear concerns and collect feedback so we can address them to
> hit that very high bar.
>
>
> ~t
>
> --
> Tianyin Xu
> University of Illinois at Urbana-Champaign
> https://tianyin.github.io/