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

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