nbd/server: CVE-2024-7409: Avoid use-after-free when closing server

Commit 3e7ef738 plugged the use-after-free of the global nbd_server
object, but overlooked a use-after-free of nbd_server->listener.
Although this race is harder to hit, notice that our shutdown path
first drops the reference count of nbd_server->listener, then triggers
actions that can result in a pending client reaching the
nbd_blockdev_client_closed() callback, which in turn calls
qio_net_listener_set_client_func on a potentially stale object.

If we know we don't want any more clients to connect, and have already
told the listener socket to shut down, then we should not be trying to
update the listener socket's associated function.

Reproducer:

> #!/usr/bin/python3
>
> import os
> from threading import Thread
>
> def start_stop():
>     while 1:
>         os.system('virsh qemu-monitor-command VM \'{"execute": "nbd-server-start",
+"arguments":{"addr":{"type":"unix","data":{"path":"/tmp/nbd-sock"}}}}\'')
>         os.system('virsh qemu-monitor-command VM \'{"execute": "nbd-server-stop"}\'')
>
> def nbd_list():
>     while 1:
>         os.system('/path/to/build/qemu-nbd -L -k /tmp/nbd-sock')
>
> def test():
>     sst = Thread(target=start_stop)
>     sst.start()
>     nlt = Thread(target=nbd_list)
>     nlt.start()
>
>     sst.join()
>     nlt.join()
>
> test()

Fixes: CVE-2024-7409
Fixes: 3e7ef738c8 ("nbd/server: CVE-2024-7409: Close stray clients at server-stop")
CC: qemu-stable@nongnu.org
Reported-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-ID: <20240822143617.800419-2-eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

blockdev-nbd.c

@@ -92,10 +92,13 @@ static void nbd_accept(QIONetListener *listener, QIOChannelSocket *cioc,
 
 static void nbd_update_server_watch(NBDServerData *s)
 {
-    if (!s->max_connections || s->connections < s->max_connections) {
-        qio_net_listener_set_client_func(s->listener, nbd_accept, NULL, NULL);
-    } else {
-        qio_net_listener_set_client_func(s->listener, NULL, NULL, NULL);
+    if (s->listener) {
+        if (!s->max_connections || s->connections < s->max_connections) {
+            qio_net_listener_set_client_func(s->listener, nbd_accept, NULL,
+                                             NULL);
+        } else {
+            qio_net_listener_set_client_func(s->listener, NULL, NULL, NULL);
+        }
     }
 }
 
@@ -113,6 +116,7 @@ static void nbd_server_free(NBDServerData *server)
      */
     qio_net_listener_disconnect(server->listener);
     object_unref(OBJECT(server->listener));
+    server->listener = NULL;
     QLIST_FOREACH_SAFE(conn, &server->conns, next, tmp) {
         qio_channel_shutdown(QIO_CHANNEL(conn->cioc), QIO_CHANNEL_SHUTDOWN_BOTH,
                              NULL);