netdev - Re: Question about seccomp / bpf

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Date:   Thu, 9 May 2019 16:30:25 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        Alexei Starovoitov <ast@...com>,
        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 09, 2019 at 01:49:25PM +0200, Daniel Borkmann wrote:
> On 05/09/2019 12:58 PM, Eric Dumazet wrote:
> > 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 )
> 
> +1, that would indeed be worth to pursue as a short term solution.

I'm not sure how that can work. seccomp's prctl accepts a list of insns.
There is no handle.
kernel can keep a hashtable of all progs ever loaded and do a search
in it before loading another one, but that's an ugly hack.
Another alternative is to attach seccomp prog to parent task
instead of N childrens.