netdev - Re: [PATCH bpf-next v2] bpf, seccomp: fix false positive preemption splat for cbpf->ebpf progs

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Date:   Thu, 21 Feb 2019 14:14:13 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Jann Horn <jannh@...gle.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andy Lutomirski <luto@...capital.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2] bpf, seccomp: fix false positive preemption
 splat for cbpf->ebpf progs

On Thu, Feb 21, 2019 at 12:36 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
> I also would like to touch on your comment:
> "A lot of changes will be needed for seccomp ebpf"
> There were two attempts to add it in the past and the patches were
> small and straightforward.

Yeah, agreed: doing it is technically easy. My concerns have mainly
revolved around avoiding increased complexity and attack surface.
There have been, for example, a lot of verifier bugs that were not
reachable through seccomp's BPF usage, given it enforcing only using a
subset of cBPF. i.e. seccomp filters couldn't be used as Spectre
gadgets, etc.

> If I recall correctly both times you nacked them because performance gains
> and ease of use arguments were not convincing enough, right?

Right. There wasn't, in my opinion enough of a performance benefit vs
just having efficient BPF to start with.

> Are you still not convinced ?

For now, yeah. I'm sure there will be some future time when a use-case
appears where gaining some special eBPF hook/feature will outweigh the
increased attack surface. I haven't seen it yet, but I'm not crazy
enough to think it'll never happen. (In fact, recently I even had
Tycho see if he could implement the recent seccomp user notification
stuff via eBPF.)

-- 
Kees Cook