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

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Message-ID: <CALCETrV9xUd3XRgobTDgVNRFY_+o=pEDkfjvuxQ7w_UyH324zA@mail.gmail.com>
Date:   Wed, 14 Feb 2018 17:25:00 +0000
From:   Andy Lutomirski <luto@...capital.net>
To:     Kees Cook <keescook@...omium.org>
Cc:     Sargun Dhillon <sargun@...gun.me>,
        Network Development <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Will Drewry <wad@...omium.org>
Subject: Re: [PATCH net-next 0/3] eBPF Seccomp filters

On Tue, Feb 13, 2018 at 3:47 PM, Kees Cook <keescook@...omium.org> wrote:
> 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.
>
> What's the reason for adding eBPF support? seccomp shouldn't need it,
> and it only makes the code more complex. I'd rather stick with cBPF
> until we have an overwhelmingly good reason to use eBPF as a "native"
> seccomp filter language.
>

I can think of two fairly strong use cases for eBPF's ability to call
functions: logging and Tycho's user notifier thing.  They let seccomp
filters *do* something synchronously, which is a better match for both
use cases than the current approach of "hey, I'd like to log this
syscall, but it's really awkward to attach other information or to
track exactly *which* filter logged what or to stack any of it".

Also, eBPF's stronger arithmetic support would allow bitops (I think),
which would make "is the nr in this list" quite a bit faster in some
cases.