netdev - Re: [PATCH net-next 0/3] eBPF Seccomp filters

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Date:   Tue, 13 Feb 2018 13:33:44 -0700
From:   Tom Hromatka <tom.hromatka@...cle.com>
To:     netdev@...r.kernel.org, sargun@...gun.me
Cc:     wad@...omium.org, Kees Cook <keescook@...omium.org>,
        daniel@...earbox.net, containers@...ts.linux-foundation.org,
        ast@...nel.org, luto@...capital.net
Subject: Re: [PATCH net-next 0/3] eBPF Seccomp filters

On Tue, Feb 13, 2018 at 7:42 AM, Sargun Dhillon <sargun@...gun.me> wrote:
> This patchset enables seccomp filters to be written in eBPF. Although,
> this patchset doesn't introduce much of the functionality enabled by
> eBPF, it lays the ground work for it.
>
> It also introduces the capability to dump eBPF filters via the PTRACE
> API in order to make it so that CHECKPOINT_RESTORE will be satisifed.
> In the attached samples, there's an example of this. One can then use
> BPF_OBJ_GET_INFO_BY_FD in order to get the actual code of the program,
> and use that at reload time.
>
> The primary reason for not adding maps support in this patchset is
> to avoid introducing new complexities around PR_SET_NO_NEW_PRIVS.
> If we have a map that the BPF program can read, it can potentially
> "change" privileges after running. It seems like doing writes only
> is safe, because it can be pure, and side effect free, and therefore
> not negatively effect PR_SET_NO_NEW_PRIVS. Nonetheless, if we come
> to an agreement, this can be in a follow-up patchset.


Coincidentally I also sent an RFC for adding eBPF hash maps to the seccomp
userspace mailing list just last week:
https://groups.google.com/forum/#!topic/libseccomp/pX6QkVF0F74

The kernel changes I proposed are in this email:
https://groups.google.com/d/msg/libseccomp/pX6QkVF0F74/ZUJlwI5qAwAJ

In that email thread, Kees requested that I try out a binary tree in cBPF
and evaluate its performance.  I just got a rough prototype working, and
while not as fast as an eBPF hash map, the cBPF binary tree was a significant
improvement over the linear list of ifs that are currently generated.  Also,
it only required changing a single function within the libseccomp libary
itself.

https://github.com/drakenclimber/libseccomp/commit/87b36369f17385f5a7a4d95101185577fbf6203b

Here are the results I am currently seeing using an in-house customer's
seccomp filter and a simplistic test program that runs getppid() thousands
of times.

Test Case                      minimum TSC ticks to make syscall
----------------------------------------------------------------
seccomp disabled                                             620
getppid() at the front of 306-syscall seccomp filter         722
getppid() in middle of 306-syscall seccomp filter           1392
getppid() at the end of the 306-syscall filter              2452
seccomp using a 306-syscall-sized EBPF hash map              800
cBPF filter using a binary tree                              922

Thanks.

Tom