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.

-- 
Kees Cook