netdev - Re: Question about seccomp / bpf

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Date:   Thu, 9 May 2019 03:52:07 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Kees Cook <keescook@...gle.com>,
        Andy Lutomirski <luto@...capital.net>,
        Jann Horn <jannh@...gle.com>
Subject: Re: Question about seccomp / bpf

On Wed, May 8, 2019 at 9:47 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Wed, May 08, 2019 at 04:17:29PM -0700, Eric Dumazet wrote:
> > On Wed, May 8, 2019 at 4:09 PM Alexei Starovoitov
> > <alexei.starovoitov@...il.com> wrote:
> > >
> > > On Wed, May 08, 2019 at 02:21:52PM -0700, Eric Dumazet wrote:
> > > > Hi Alexei and Daniel
> > > >
> > > > I have a question about seccomp.
> > > >
> > > > It seems that after this patch, seccomp no longer needs a helper
> > > > (seccomp_bpf_load())
> > > >
> > > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=bd4cf0ed331a275e9bf5a49e6d0fd55dffc551b8
> > > >
> > > > Are we detecting that a particular JIT code needs to call at least one
> > > > function from the kernel at all ?
> > >
> > > Currently we don't track such things and trying very hard to avoid
> > > any special cases for classic vs extended.
> > >
> > > > If the filter contains self-contained code (no call, just inline
> > > > code), then we could use any room in whole vmalloc space,
> > > > not only from the modules (which is something like 2GB total on x86_64)
> > >
> > > I believe there was an effort to make bpf progs and other executable things
> > > to be everywhere too, but I lost the track of it.
> > > It's not that hard to tweak x64 jit to emit 64-bit calls to helpers
> > > when delta between call insn and a helper is more than 32-bit that fits
> > > into call insn. iirc there was even such patch floating around.
> > >
> > > but what motivated you question? do you see 2GB space being full?!
> >
> >
> > A customer seems to hit the limit, with about 75,000 threads,
> > each one having a seccomp filter with 6 pages (plus one guard page
> > given by vmalloc)
>
> Since cbpf doesn't have "fd as a program" concept I suspect
> the same program was loaded 75k times. What a waste of kernel memory.
> And, no, we're not going to extend or fix cbpf for this.
> cbpf is frozen. seccomp needs to start using ebpf.
> It can have one program to secure all threads.
> If necessary single program can be customized via bpf maps
> for each thread.

Yes,  docker seems to have a very generic implementation and  should
probably be fixed
( https://github.com/moby/moby/blob/v17.03.2-ce/profiles/seccomp/seccomp.go )