virtio-serial: fix heapover-flow

Check device having the feature of VIRTIO_CONSOLE_F_EMERG_WRITE before
get config->emerg_wr. It is neccessary because sizeof(virtio_console_config)
is 8 byte if VirtIOSerial doesn't have the feature of
VIRTIO_CONSOLE_F_EMERG_WRITE(see virtio_serial_device_realize),
read/write emerg_wr will lead to heap-over-flow.
Signed-off-by: Nlinzhecheng <linzhecheng@huawei.com>
Message-Id: <20180328133435.20112-1-linzhecheng@huawei.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/char/virtio-serial-bus.c

@@ -580,13 +580,16 @@ static void set_config(VirtIODevice *vdev, const uint8_t *config_data)
     VirtIOSerial *vser = VIRTIO_SERIAL(vdev);
     struct virtio_console_config *config =
         (struct virtio_console_config *)config_data;
-    uint8_t emerg_wr_lo = le32_to_cpu(config->emerg_wr);
     VirtIOSerialPort *port = find_first_connected_console(vser);
     VirtIOSerialPortClass *vsc;
+    uint8_t emerg_wr_lo;
 
-    if (!config->emerg_wr) {
+    if (!virtio_has_feature(vser->host_features,
+        VIRTIO_CONSOLE_F_EMERG_WRITE) || !config->emerg_wr) {
         return;
     }
+
+    emerg_wr_lo = le32_to_cpu(config->emerg_wr);
     /* Make sure we don't misdetect an emergency write when the guest
      * does a short config write after an emergency write. */
     config->emerg_wr = 0;