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

[Christian Brauner]

On June 9, 2020 10:55:42 PM GMT+02:00, Kees Cook <keescook@xxxxxxxxxxxx> 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? :)

Christian