Re: [PATCH v3 4/4] seccomp: add support for passing fds via USER_NOTIF

[Jann Horn]

On Sat, Jun 2, 2018 at 2:58 PM Tycho Andersen <tycho@xxxxxxxx> 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.
>
> Still, the vast majority of interesting cases are covered by this API, so
> perhaps it is Enough.
>
> I've left it as a separate commit for two reasons:
>   * It illustrates the way in which we would grow struct seccomp_notif and
>     struct seccomp_notif_resp without using netlink
>   * It shows just how little code is needed to accomplish this :)
[...]
> +               fd = get_unused_fd_flags(n.flags);

Here, you're using n.flags in a context where it will be tested
against O_CLOEXEC to determine whether the new fd should be
close-on-exec.

[...]
> +               /*
> +                * This is a little hokey: we need a real fget() (i.e. not
> +                * __fget_light(), which is what fdget does), but we also need
> +                * the flags from strcut fd. So, we get it, put it, and get it
> +                * again for real.
> +                */
> +               fd = fdget(resp.fd);
> +               knotif->flags = fd.flags;
> +               fdput(fd);
> +
> +               knotif->file = fget(resp.fd);
> +               if (!knotif->file) {
> +                       ret = -EBADF;
> +                       goto out;
> +               }

But here fd.flags contains the low 2 bits of the return value of
__fget_light, which are either 0 or FDPUT_FPUT (encoded as 1). This
flag states whether fdget() took a reference on the file, which is
mostly equivalent to "is the current process multithreaded?". (This is
the reason why fdget returns flags and fget doesn't - the flag from
fdget is to decide whether you'll need an fput(), which is
unconditional for fget().)

Apart from this issue, I think that in general, it's probably not a
good idea to copy the close-on-exec flag from the fd in the
supervising process - the supervising process might want all the fds
it is working with to be O_CLOEXEC independent of whether the
supervised process wants an O_CLOEXEC fd. It might make sense to add a
field for this to struct seccomp_notif_resp instead.