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

[Kees Cook]

On Wed, Feb 14, 2018 at 9:19 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Wed, Feb 14, 2018 at 3:29 PM, Tycho Andersen <tycho@xxxxxxxx> 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@xxxxxxxx> 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...

-Kees

-- 
Kees Cook
Pixel Security