netdev - Re: Question about seccomp / bpf

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Thu, 9 May 2019 16:50:12 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        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 9, 2019 at 4:30 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> 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.

I guess that if such a hack is doable and can save 2GB of memory, then
it is an acceptable one.


> Another alternative is to attach seccomp prog to parent task
> instead of N childrens.

seccomp filters are stacked, the parent(s) filter(s) might be very different.