netdev - Re: Question about seccomp / bpf

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Date:   Thu, 9 May 2019 13:49:25 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Eric Dumazet <edumazet@...gle.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     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 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.