On Wed, Oct 31, 2018 at 3:10 PM, Oleg Nesterov <o...@redhat.com> wrote: > On 10/31, Daniel Colascione wrote: >> >> > perhaps it would be simpler to do >> > >> > my_cred = override_creds(file->f_cred); >> > kill_pid(...); >> > revert_creds(my_cred); >> >> Thanks for the suggestion. That looks neat, but it's not quite enough. >> The problem is that check_kill_permission looks for >> same_thread_group(current, t) _before_ checking kill_of_by_cred, > > Yes, you are right. > > Looks like kill_pid_info_as_cred() can find another user, but probably > it needs some changes with or without /proc/pid/kill ... > >> There's another problem though: say we open /proc/pid/5/kill *, with >> proc 5 being an ordinary unprivileged process, e.g., the shell. At >> open(2) time, the access check passes. Now suppose PID 5 execve(2)s >> into a setuid process. The kill FD is still open, so the kill FD's >> holder can send a signal > > Confused... why? kill_ok_by_cred() should fail?
Not if we don't run it. :-) I thought you were proposing that we do *all* access checks in open() and let write() succeed unconditionally, since that's the model that a lot of FD-mediated resources (like regular files) use. (MAC notwithstanding.) Anyway, I sent a v2 patch that I think closes the hole another way. In v2, we just require that the real user ID that opens a /proc/pid/kill file is the same one that writes to it. It successfully blocks the setuid attack above while preserving all the write-time permission checks and keeping the close correspondence between write()-on-proc-pid-kill-fd and kill(2). Can you think of any situation where this scheme breaks? I *think* comparing struct user addresses instead of numeric UIDs will protect the check against user namespace shenanigans.
