netdev - Re: [net-next v3 0/2] eBPF seccomp filters

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Date:   Tue, 27 Feb 2018 22:58:11 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Kees Cook <keescook@...omium.org>
Cc:     chris hyser <chris.hyser@...cle.com>,
        Andy Lutomirski <luto@...capital.net>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Netdev <netdev@...r.kernel.org>, Will Drewry <wad@...omium.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Sargun Dhillon <sargun@...gun.me>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Subject: Re: [net-next v3 0/2] eBPF seccomp filters

On 02/27/2018 05:59 PM, chris hyser wrote:
> On 02/27/2018 11:00 AM, Kees Cook wrote:
>> On Tue, Feb 27, 2018 at 6:53 AM, chris hyser <chris.hyser@...cle.com> wrote:
>>> On 02/26/2018 11:38 PM, Kees Cook wrote:
>>>> On Mon, Feb 26, 2018 at 8:19 PM, Andy Lutomirski <luto@...capital.net>
>>>> wrote:
>>>>>
>>>>> 3. Straight-up bugs.  Those are exactly as problematic as verifier
>>>>> bugs in any other unprivileged eBPF program type, right?  I don't see
>>>>> why seccomp is special here.
>>>>
>>>> My concern is more about unintended design mistakes or other feature
>>>> creep with side-effects, especially when it comes to privileges and
>>>> synchronization. Getting no-new-privs done correctly, for example,
>>>> took some careful thought and discussion, and I'm shy from how painful
>>>> TSYNC was on the process locking side, and eBPF has had some rather
>>>> ugly flaws in the past (and recently: it was nice to be able to say
>>>> for Spectre that seccomp filters couldn't be constructed to make
>>>> attacks but eBPF could). Adding the complexity needs to be worth the

Well, not really. One part of all the Spectre mitigations that went upstream
from BPF side was to have an option to remove interpreter entirely and that
also relates to seccomp eventually. But other than that an attacker might
potentially find as well useful gadgets inside seccomp or any other code
that is inside the kernel, so it's not a strict necessity either.

>>>> gain. I'm on board for doing it, I just want to be careful. :)
>>>
>>> Another option might be to remove c/eBPF from the equation all together.
>>> c/eBPF allows flexibility and that almost always comes at the cost of
>>> additional security risk. Seccomp is for enhanced security yes? How about a
>>> new seccomp mode that passes in something like a bit vector or hashmap for
>>> "simple" white/black list checks validated by kernel code, versus user
>>> provided interpreted code? Of course this removes a fair number of things
>>> you can currently do or would be able to do with eBPF. Of course, restated
>>> from a security point of view, this removes a fair number of things an
>>> _attacker_ can do. Presumably the performance improvement would also be
>>> significant.

Good luck with not breaking existing applications relying on seccomp out
there.

>>> Is this an idea worth prototyping?
>>
>> That was the original prototype for seccomp-filter. :) The discussion
>> around that from years ago basically boiled down to it being
>> inflexible. Given all the things people want to do at syscall time,
>> that continues to be true. So true, in fact, that here we are now,
>> trying to move to eBPF from cBPF. ;)

Right, agree. cBPF is also pretty much frozen these days and aside from
that, seccomp/BPF also just uses a proper subset of it. I wouldn't mind
doing something similar for eBPF side as long as this is reasonably
maintainable and not making BPF core more complex, but most of it can
already be set in the verifier anyway based on prog type. Note, that
performance of seccomp/BPF is definitely a demand as well which is why
people still extend the old remaining cBPF JITs today such that it can
be JITed also from there.

> I will try to find that discussion. As someone pointed out here though, eBPF is being used by more and more people in areas where security is not the primary concern. Differing objectives will make this a long term continuing issue. We ourselves were looking at eBPF simply as a means to use a hashmap for a white/blacklist, i.e. performance not flexibility.

Not really, security of verifier and BPF infra in general is on the top
of the list, it's fundamental to the underlying concept and just because
it is heavily used also in tracing and networking, it only shows that the
concept is highly flexible that it can be applied in multiple areas.