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

From: Kees Cook <keescook@chromium.org>
To: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Tianyin Xu <tyxu@illinois.edu>,
	Tycho Andersen <tycho@tycho.pizza>,
	Andy Lutomirski <luto@kernel.org>,
	YiFei Zhu <zhuyifei1999@gmail.com>,
	"containers@lists.linux.dev" <containers@lists.linux.dev>,
	bpf <bpf@vger.kernel.org>, "Zhu, YiFei" <yifeifz2@illinois.edu>,
	LSM List <linux-security-module@vger.kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Andrea Arcangeli <aarcange@redhat.com>,
	"Kuo, Hsuan-Chi" <hckuo2@illinois.edu>,
	Claudio Canella <claudio.canella@iaik.tugraz.at>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Daniel Gruss <daniel.gruss@iaik.tugraz.at>,
	Dimitrios Skarlatos <dskarlat@cs.cmu.edu>,
	Giuseppe Scrivano <gscrivan@redhat.com>,
	Hubertus Franke <frankeh@us.ibm.com>,
	Jann Horn <jannh@google.com>,
	"Jia, Jinghao" <jinghao7@illinois.edu>,
	"Torrellas, Josep" <torrella@illinois.edu>,
	Sargun Dhillon <sargun@sargun.me>,
	Tobin Feldman-Fitzthum <tobin@ibm.com>,
	Tom Hromatka <tom.hromatka@oracle.com>,
	Will Drewry <wad@chromium.org>
Subject: Re: [RFC PATCH bpf-next seccomp 00/12] eBPF seccomp filters
Date: Tue, 1 Jun 2021 12:55:10 -0700	[thread overview]
Message-ID: <202106011244.76762C210@keescook> (raw)
In-Reply-To: <20210520085613.gvshk4jffmzggvsm@wittgenstein>

On Thu, May 20, 2021 at 10:56:13AM +0200, Christian Brauner 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.

Before I dive in, I do want to say that this is very interesting work.
Thanks for working on it, even if we're all so grumpy about accepting
it. :)

> > > > > >>> 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*.

Likely everyone is aware, but I'll point out for anyone new reading this
thread: seccomp uses eBPF under the hood: all the cBPF is transformed to
eBPF at filter attach time. But yes, I get the point: using the _entire_
eBPF language. Though I'd remind folks that seccomp doesn't even use
the entire cBPF language...

> [...] 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.

This is the blocker as far as I'm concerned: there is no story for
unprivileged eBPF. And even IF there was a story there, I find the rate
of security-related flaws in eBPF to be way too high for a sandboxing
primitive to depend on. There have been around a dozen a year for the
last 4 years:

$ git log --oneline --no-merges --pretty=format:'%as %h %s' \
   -i -E \ --all-match --grep '^Fixes:' --grep \
   '(over|under)flow|\bleak|escalat|expos(e[ds]?|ure)\b|use[- ]?after[- ]?free' \
   -- kernel/bpf/ | cut -d- -f1 | sort | uniq -c
      4 2015
      4 2016
     13 2017
     16 2018
     18 2019
     12 2020
      6 2021

I just can't bring myself to accept that level of risk for seccomp. (And
yes, this might be mitigated by blocking the bpf() syscall within a
filter, but then eBPF seccomp would become kind of useless inside a
container launcher, etc etc.)

-Kees

-- 
Kees Cook