netdev - Re: Question about seccomp / bpf

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Date:   Thu, 9 May 2019 03:58:53 -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>, Will Drewry <wad@...omium.org>
Subject: Re: Question about seccomp / bpf

On Thu, May 9, 2019 at 3:52 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> 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 )

Even if the seccomp program was optimized to a few bytes, it would
still consume at least 2 pages in module vmalloc space,
so the limit in number of concurrent programs would be around 262,144

We might ask seccomp guys to detect that the same program is used, by
maintaining a hash of already loaded ones.
( I see struct seccomp_filter has a @usage refcount_t )