Re: [PATCH v6 3/5] seccomp: add a way to get a listener fd from ptrace

[Andy Lutomirski <luto@amacapital.net>]

On Thu, Sep 6, 2018 at 8:28 AM, Tycho Andersen <tycho@tycho.ws> wrote:
> As an alternative to SECCOMP_FILTER_FLAG_GET_LISTENER, perhaps a ptrace()
> version which can acquire filters is useful. There are at least two reasons
> this is preferable, even though it uses ptrace:
>
> 1. You can control tasks that aren't cooperating with you
> 2. You can control tasks whose filters block sendmsg() and socket(); if the
>    task installs a filter which blocks these calls, there's no way with
>    SECCOMP_FILTER_FLAG_GET_LISTENER to get the fd out to the privileged task.

Hmm.  I contemplated this a bit and looked at your example a bit, and
I have a few thoughts:

 - What happens if you nest code like your sample?  That is, if you
are already in some container that is seccomped and there's a
listener, can you even run your sample?

 - Is there any association between the filter layer that uses the
USER_NOTIF return and the listener?  How would this API express such a
relationship?

I realize that my dream of how this should all work requires eBPF and
BPF_CALL, so it may not be viable right now, but I'd like a better
understanding of how this all fits together.

Also, I think that it's not strictly true that a filter that blocks
sendmsg() is problematic.  You could clone a thread, call seccomp() in
that thread, then get a listener, then execve().  Or we could have a
seccomp() mode that adds a filter but only kicks in after execve().
The latter could be generally useful.

--Andy