qemu-kvm/kvm-nbd-server-CVE-2024-7409-Avoid-use-after-free-when-c.patch

From 3732f1491d8981e85f699fcd125d903aba77fa32 Mon Sep 17 00:00:00 2001
From: Eric Blake <eblake@redhat.com>
Date: Thu, 22 Aug 2024 09:35:29 -0500
Subject: [PATCH] nbd/server: CVE-2024-7409: Avoid use-after-free when closing
 server

RH-Author: Eric Blake <eblake@redhat.com>
RH-MergeRequest: 267: nbd/server: CVE-2024-7409: Avoid use-after-free when closing server
RH-Jira: RHEL-52599
RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
RH-Acked-by: Hanna Czenczek <hreitz@redhat.com>
RH-Commit: e7d52e5d1372eaec00325d4854772ee78fe650b7 (ebblake/centos-qemu-kvm)

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>

(cherry picked from commit 3874f5f73c441c52f1c699c848d463b0eda01e4c)
Jira: https://issues.redhat.com/browse/RHEL-52599
Signed-off-by: Eric Blake <eblake@redhat.com>
---
 blockdev-nbd.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/blockdev-nbd.c b/blockdev-nbd.c
index f73409ae49..b36f41b7c5 100644
--- a/blockdev-nbd.c
+++ b/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);
-- 
2.39.3