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 16:22:34 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Jann Horn <jannh@...gle.com>,
        Daniel Borkmann <daniel@...earbox.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 2:14 PM Kees Cook <keescook@...omium.org> wrote:
>
> 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.)
>

I consider the potential for much improved performance to be a
maybe-good-enough argument.  The down side is that there are programs
that load cBPF seccomp filters from inside a sandbox, and being able
to load eBPF from inside a sandbox is potentially undesirable.