linux-kernel - Re: [RFC 1/3] seccomp: add a return code to trap to userspace

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Date:   Mon, 26 Feb 2018 19:27:51 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Kees Cook <keescook@...omium.org>
Cc:     Tycho Andersen <tycho@...ho.ws>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Tyler Hicks <tyhicks@...onical.com>,
        Akihiro Suda <suda.akihiro@....ntt.co.jp>,
        Tom Hromatka <tom.hromatka@...cle.com>,
        Sargun Dhillon <sargun@...gun.me>,
        Paul Moore <pmoore@...hat.com>, ast@...nel.org
Subject: Re: [RFC 1/3] seccomp: add a return code to trap to userspace



> On Feb 26, 2018, at 4:49 PM, Kees Cook <keescook@...omium.org> wrote:
> 
>> On Wed, Feb 14, 2018 at 9:19 AM, Andy Lutomirski <luto@...capital.net> wrote:
>>> On Wed, Feb 14, 2018 at 3:29 PM, Tycho Andersen <tycho@...ho.ws> wrote:
>>>> On Tue, Feb 13, 2018 at 01:09:20PM -0800, Kees Cook wrote:
>>>> On Sun, Feb 4, 2018 at 2:49 AM, Tycho Andersen <tycho@...ho.ws> wrote:
>>>> I wonder if this communication should be netlink, which gives a more
>>>> well-structured way to describe what's on the wire? The reason I ask
>>>> is because if we ever change the seccomp_data structure, we'll now
>>>> have two places where we need to deal with it (the first being within
>>>> the BPF itself). My initial idea was to prefix the communication with
>>>> a size field, then send the structure, and then I had nightmares, and
>>>> realized this was basically netlink reinvented.
>>> 
>>> I suggested netlink in LA, and everyone (especially Andy) groaned very
>>> loudly :). I'm happy to switch it to netlink if you like, although i
>>> think memcpy() of structs should be safe here, since the return value
>>> from read or write can indicate the size of things.
>> 
>> I could easily get on board with "netlink" (i.e. NLA) messages sent
>> over an fd.  I will object strongly to the use of netlink *sockets*.
> 
> Yeah, I was thinking NLA over the fd; not a netlink socket.
> 
>>>> An ERRNO filter would block a USER_NOTIF because it's unconditional.
>>>> TRACE could be either, USER_NOTIF could be either.
>>>> 
>>>> This means TRACE rules would be bumped by a USER_NOTIF... hmm.
>>> 
>>> Yes, I didn't exactly know what to do here. ERRNO, TRAP, and KILL all
>>> seemed more important than USER_NOTIF, but TRACE didn't. I don't have
>>> a strong opinion about what to do here, because users can adjust their
>>> filters accordingly. Let me know what you prefer.
>> 
>> If we switched to eBPF functions, this whole issue goes away.
> 
> Yeah, though we'd still need some kind of "wait for answer" eBPF
> function. It feels wrong to re-use maps for that...
> 

BPF_CALL.

Alexei, can we make it so that each bpf program type can easily limit which BPF_CALL helpers can be use and allow bpf program types to add their own helpers?c