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

[Sargun Dhillon <sargun@sargun.me>]

On Thu, May 20, 2021 at 1:22 AM Tianyin Xu <tyxu@illinois.edu> wrote:
>
> On Mon, May 17, 2021 at 12:08 PM Sargun Dhillon <sargun@sargun.me> wrote:
> >
> > 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.
>
> Thanks for the pointer! But, I don’t think this can fundamentally
> eliminate the performance gap between the notifiers and the ebpf
> filters. IMHO, the additional context switches of user notifiers make
> the difference.
>
I mean, I still think it can be closed. Or at least get better. I've
thought about
working on performance improvements, but they're lower on the list
than functionality changes.

> >
> > 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.
> >
>
> Apologize that I don’t fully understand your proposal. My
> understanding about io_uring is that it allows you to amortize the
> cost of context switch but not eliminate it, unless you are willing to
> dedicate a core for it. I still believe that, even with io_uring, user
> notifiers are going to be much slower than eBPF filters.
The notifier gets significantly slower as a function of the notifications. If
you have a large number of notifications in flight, or if you're trying to
concurrently handle a large number of notifications, it gets slower. This
is where something like io_uring is super useful in terms of reducing
wakeups.

Also, in the original futex2 patches, it had a mechanism to better handle
(scheduling) of notifier like cases[1]. If the seccomp notifier did a similar
thing, we could see better performance.

>
> Btw, our patches are based on your patch set (thank you!). Are you
> using user notifiers (with your improved version?) these days? It will
> be nice to hear your opinions on ebpf filters.
>
I'm so glad that someone is picking up the work on this.

> > >
> > >
> > > > >> 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.
> > > >
So, we actually have a case where we want to inspect an argument --
We want to look at the FD number that's passed to the sendmsg syscall, and then
see if that's an AF_INET socket, and if it is, then pass back to
notifier, otherwise
allow it to continue through. This is an area where I can see eBPF being
very useful.

> > > > >>
> > > > >> 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://urldefense.com/v3/__https://tianyin.github.io/__;!!DZ3fjg!o4__Ob32oapUDg9_f6hzksoFiX9517CJ5-w8qtG9i-WKFs_xWbGQfUHpLjHjCddw$
>

[1]: https://lore.kernel.org/lkml/20210215152404.250281-1-andrealmeid@collabora.com/T/