netdev - refcounting on dgram Unix sockets for poll(POLLOUT)

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Date:	Wed, 8 Dec 2010 21:01:00 +0000
From:	Alban Crequy <alban.crequy@...labora.co.uk>
To:	netdev@...r.kernel.org
Subject: refcounting on dgram Unix sockets for poll(POLLOUT)

Hi,

When a connected datagram Unix socket is polled for POLLOUT events, the
poller is added in the wait_queue_head_t of the "server" socket:

net/unix/af_unix.c::unix_dgram_poll()
  other = unix_peer_get(sk);
  if (other) {
    if (unix_peer(other) != sk) {
      sock_poll_wait(file, &unix_sk(other)->peer_wait,
                     wait);
      ...

I wonder what prevent the "server" socket ("other") to be released
while the poller is still waiting for POLLOUT events.

There is a reference taken on the "server" socket when the client
connects:
net/unix/af_unix.c::unix_dgram_connect()
  other = unix_find_other(net, sunaddr, alen, sock->type,
            hash, &err);

But that reference could be released when the client socket
disconnects from another thread in one of the 3 possible locations:

1. unix_dgram_connect() when connecting to a different socket
     if (other != old_peer)
       unix_dgram_disconnected(sk, old_peer);
     sock_put(old_peer);

2. unix_dgram_sendmsg() when the server socket is SOCK_DEAD:
     unix_dgram_disconnected(sk, other);
     sock_put(other);

3. unix_release_sock() when the client socket is released:
     skpair = unix_peer(sk);
     if (skpair != NULL) {
       sock_put(skpair); /* It may now die */

I tried to release all the references to server_sockfd with
close(server_sockfd) on the server thread and with
connect(client_sockfd) to a different socket while client_sockfd is
being polled for POLLOUT events in a different thread, hoping to crash
the poller with the stack:
free_poll_entry()->remove_wait_queue()->spin_lock_irqsave()
But I didn't manage to crash the kernel.

Am I missing something? Is there another reference taken on
server_sockfd to protect the kernel from this scenario?

And btw, what is the test (unix_peer(other) != sk) in unix_dgram_poll()?

Alban
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html