linux-kernel - Re: [PATCH v6 4/5] seccomp: add support for passing fds via USER

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Date:   Thu, 13 Sep 2018 03:25:44 -0600
From:   Tycho Andersen <tycho@...ho.ws>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Kees Cook <keescook@...omium.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Linux API <linux-api@...r.kernel.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>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH v6 4/5] seccomp: add support for passing fds via
 USER_NOTIF

On Wed, Sep 12, 2018 at 04:52:38PM -0700, Andy Lutomirski wrote:
> On Thu, Sep 6, 2018 at 8:28 AM, Tycho Andersen <tycho@...ho.ws> wrote:
> > The idea here is that the userspace handler should be able to pass an fd
> > back to the trapped task, for example so it can be returned from socket().
> >
> > I've proposed one API here, but I'm open to other options. In particular,
> > this only lets you return an fd from a syscall, which may not be enough in
> > all cases. For example, if an fd is written to an output parameter instead
> > of returned, the current API can't handle this. Another case is that
> > netlink takes as input fds sometimes (IFLA_NET_NS_FD, e.g.). If netlink
> > ever decides to install an fd and output it, we wouldn't be able to handle
> > this either.
> 
> An alternative could be to have an API (an ioctl on the listener,
> perhaps) that just copies an fd into the tracee.  There would be the
> obvious set of options: do we replace an existing fd or allocate a new
> one, and is it CLOEXEC.  Then the tracer could add an fd and then
> return it just like it's a regular number.
> 
> I feel like this would be more flexible and conceptually simpler, but
> maybe a little slower for the common cases.  What do you think?

Yes, I like this. It also future (or current-) proofs the API against
instances where we return an FD in a structure and not via the return
code of the syscall.

Tycho