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? :)
Christian