[Qemu-devel] [PATCH] net: prevent sending packets while guest is stopped

[Stefan Hajnoczi <stefanha@redhat.com>]

Do not modify guest memory or devices when the guest is stopped.
Currently the netdevs still send packets while the guest is stopped if
their file descriptor was being monitored for write (e.g. the socket
write buffer filled before the guest was stopped).

Netdevs call qemu_flush_queued_packets() when the file descriptor
becomes writable again.  Don't resume packet processing when this
happens.

Instead we flush queues when the guest resumes.

Cc: qemu-stable@nongnu.org
Reported-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
Note this fixes the transmit side.  The receive side was recently fixed in
"net: Forbid dealing with packets when VM is not running".

 net/net.c | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/net/net.c b/net/net.c
index 6d930ea..74ec07a 100644
--- a/net/net.c
+++ b/net/net.c
@@ -47,6 +47,7 @@
 # define CONFIG_NET_BRIDGE
 #endif
 
+static VMChangeStateEntry *net_change_state_entry;
 static QTAILQ_HEAD(, NetClientState) net_clients;
 
 const char *host_net_devices[] = {
@@ -506,6 +507,11 @@ void qemu_purge_queued_packets(NetClientState *nc)
 
 void qemu_flush_queued_packets(NetClientState *nc)
 {
+    /* Guest memory and devices must not be modified while stopped */
+    if (!runstate_is_running()) {
+        return;
+    }
+
     nc->receive_disabled = 0;
 
     if (nc->peer && nc->peer->info->type == NET_CLIENT_OPTIONS_KIND_HUBPORT) {
@@ -1168,6 +1174,25 @@ void qmp_set_link(const char *name, bool up, Error **errp)
     }
 }
 
+/* Kick net clients when guest resumes.  If a file descriptor was monitored for
+ * writing before the guest was stopped, there will be nothing monitoring it
+ * right now so a kick is required to get packets flowing again.
+ */
+static void net_vm_change_state_handler(void *opaque, int running,
+                                        RunState state)
+{
+    NetClientState *nc;
+    NetClientState *tmp;
+
+    if (!running) {
+        return;
+    }
+
+    QTAILQ_FOREACH_SAFE(nc, &net_clients, next, tmp) {
+        qemu_flush_queued_packets(nc);
+    }
+}
+
 void net_cleanup(void)
 {
     NetClientState *nc;
@@ -1183,6 +1208,8 @@ void net_cleanup(void)
             qemu_del_net_client(nc);
         }
     }
+
+    qemu_del_vm_change_state_handler(net_change_state_entry);
 }
 
 void net_check_clients(void)
@@ -1268,6 +1295,9 @@ int net_init_clients(void)
 #endif
     }
 
+    net_change_state_entry =
+        qemu_add_vm_change_state_handler(net_vm_change_state_handler, NULL);
+
     QTAILQ_INIT(&net_clients);
 
     if (qemu_opts_foreach(qemu_find_opts("netdev"), net_init_netdev, NULL, 1) == -1)
-- 
1.9.3