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.
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