[PATCH v6 4/5] seccomp: add support for passing fds via USER_NOTIF

[Tycho Andersen]

On Fri, Sep 21, 2018 at 11:27:59AM -0700, Andy Lutomirski wrote:
> On Fri, Sep 21, 2018 at 6:39 AM Tycho Andersen <tycho at tycho.ws> wrote:
> >
> > On Thu, Sep 20, 2018 at 07:18:45PM -0700, Andy Lutomirski wrote:
> > >
> > > I think we just want the operation to cover all the cases.  Let PUT_FD
> > > take a source fd and a dest fd.  If the source fd is -1, the dest is
> > > closed.  If the source is -1 and the dest is -1, return -EINVAL.  If
> > > the dest is -1, allocate an fd.  If the dest is >= 0, work like
> > > dup2().  (The latter could be necessary to emulate things like, say,
> > > dup2 :))
> >
> > ...then if we're going to allow overwriting fds, we'd need to lift out
> > the logic from do_dup2 somewhere? Is this getting too complicated? :)
> >
> 
> fds are complicated :-p

:D

> More seriously, though, I think it's okay if we don't support
> everything out of the box.  getting the general semantics I suggested
> is kind of nice because the resulting API is conceptually simple, even
> if it encapsulates three cases.  But I'd be okay with only supporting
> add-an-fd-at-an-unused-position and delete-an-fd out of the box --
> more can be added if there's demand.

It's the delete/replace-an-fd one that has me worried. Anyway, I'll
take a look and see what I can figure out.

> But I think that exposing an operation that allocates and reserves an
> fd without putting anything in the slot is awkward, and it opens us up
> to weird corner cases becoming visible that are currently there but
> mostly hidden.  For example, what happens if someone overwrites a
> reserved fd with dup2()?  (The answer is apparently -EBUSY -- see the
> big comment in do_dup2() in fs/file.c.)  But there's a more
> significant nastiness: what happens if someone abuses your new
> mechanism to overwrite a reserved fd that belongs to a different
> thread?  It looks like you'll hit the BUG_ON(fdt->fd[fd] != NULL); in
> __fd_install().  So unless you actually track which unused fds you own
> and enforce that the final installation installs in the right slot,
> you have a problem.
> 
> BTW, socketpair() isn't the only thing that can add two fds.
> recvmsg() can, too, as can pipe() and pipe2().  Some of the DRM ioctls
> may as well for all I know.  But socketpair(), pipe(), and recvmsg()
> can be credibly emulated by adding each fd in sequence and then
> deleting them all of one fails.  Sure, this could race against dup2(),
> but I'm not sure we care.

Yup agreed. We need to do the install when the ioctl() is called.

Tycho