Re: [PATCH v3 1/4] fs, net: Standardize on file_receive helper to move fds across processes

Christian Brauner
2020-06-10T09:31:06Z

On Tue, Jun 09, 2020 at 11:27:30PM +0200, Christian Brauner wrote:
> On June 9, 2020 10:55:42 PM GMT+02:00, Kees Cook <keescook@chromium.org> wrote:
> >On Tue, Jun 09, 2020 at 10:03:46PM +0200, Christian Brauner wrote:
> >> I'm looking at __scm_install_fd() and I wonder what specifically you
> >> mean by that? The put_user() seems to be placed such that the install
> >> occurrs only if it succeeded. Sure, it only handles a single fd but
> >> whatever. Userspace knows that already. Just look at systemd when a
> >msg
> >> fails:
> >>
> >> void cmsg_close_all(struct msghdr *mh) {
> >> struct cmsghdr *cmsg;
> >>
> >> assert(mh);
> >>
> >> CMSG_FOREACH(cmsg, mh)
> >> if (cmsg->cmsg_level == SOL_SOCKET && cmsg->cmsg_type
> >== SCM_RIGHTS)
> >> close_many((int*) CMSG_DATA(cmsg),
> >(cmsg->cmsg_len - CMSG_LEN(0)) / sizeof(int));
> >> }
> >>
> >> The only reasonable scenario for this whole mess I can think of is sm
> >like (pseudo code):
> >>
> >> fd_install_received(int fd, struct file *file)
> >> {
> >> sock = sock_from_file(fd, &err);
> >> if (sock) {
> >> sock_update_netprioidx(&sock->sk->sk_cgrp_data);
> >> sock_update_classid(&sock->sk->sk_cgrp_data);
> >> }
> >>
> >> fd_install();
> >> }
> >>
> >> error = 0;
> >> fdarray = malloc(fdmax);
> >> for (i = 0; i < fdmax; i++) {
> >> fdarray[i] = get_unused_fd_flags(o_flags);
> >> if (fdarray[i] < 0) {
> >> error = -EBADF;
> >> break;
> >> }
> >>
> >> error = security_file_receive(file);
> >> if (error)
> >> break;
> >>
> >> error = put_user(fd_array[i], ufd);
> >> if (error)
> >> break;
> >> }
> >>
> >> for (i = 0; i < fdmax; i++) {
> >> if (error) {
> >> /* ignore errors */
> >> put_user(-EBADF, ufd); /* If this put_user() fails and the first
> >one succeeded userspace might now close an fd it didn't intend to. */
> >> put_unused_fd(fdarray[i]);
> >> } else {
> >> fd_install_received(fdarray[i], file);
> >> }
> >> }
> >
> >I see 4 cases of the same code pattern (get_unused_fd_flags(),
> >sock_update_*(), fd_install()), one of them has this difficult
> >put_user()
> >in the middle, and one of them has a potential replace_fd() instead of
> >the get_used/fd_install. So, to me, it makes sense to have a helper
> >that
> >encapsulates the common work that each of those call sites has to do,
> >which I keep cringing at all these suggestions that leave portions of
> >it
> >outside the helper.
> >
> >If it's too ugly to keep the put_user() in the helper, then we can try
> >what was suggested earlier, and just totally rework the failure path
> >for
> >SCM_RIGHTS.
> >
> >LOL. And while we were debating this, hch just went and cleaned stuff
> >up:
> >
> >2618d530dd8b ("net/scm: cleanup scm_detach_fds")
> >
> >So, um, yeah, now my proposal is actually even closer to what we
> >already
> >have there. We just add the replace_fd() logic to __scm_install_fd()
> >and
> >we're done with it.
>
> Cool, you have a link? :)

For the record, as I didn't see this yesterday since I was already
looking at a kernel with Christoph's changes. His changes just move the
logic that was already there before into a separate helper.

So effectively nothing has changed semantically in the scm code at all.

This is why I was asking yesterday what you meant by reworking the scm
code's put_user() logic as it seems obviously correct as it is done now.

Christian