[client] Fix nil pointer panic in device and engine code (#5287)

client/iface/device/device_filter.go

@@ -29,8 +29,9 @@ type PacketFilter interface {
 type FilteredDevice struct {
 	tun.Device
 
-	filter PacketFilter
-	mutex  sync.RWMutex
+	filter    PacketFilter
+	mutex     sync.RWMutex
+	closeOnce sync.Once
 }
 
 // newDeviceFilter constructor function
@@ -40,6 +41,20 @@ func newDeviceFilter(device tun.Device) *FilteredDevice {
 	}
 }
 
+// Close closes the underlying tun device exactly once.
+// wireguard-go's netTun.Close() panics on double-close due to a bare close(channel),
+// and multiple code paths can trigger Close on the same device.
+func (d *FilteredDevice) Close() error {
+	var err error
+	d.closeOnce.Do(func() {
+		err = d.Device.Close()
+	})
+	if err != nil {
+		return err
+	}
+	return nil
+}
+
 // Read wraps read method with filtering feature
 func (d *FilteredDevice) Read(bufs [][]byte, sizes []int, offset int) (n int, err error) {
 	if n, err = d.Device.Read(bufs, sizes, offset); err != nil {

client/iface/device/device_netstack.go

@@ -82,7 +82,9 @@ func (t *TunNetstackDevice) create() (WGConfigurer, error) {
 	t.configurer = configurer.NewUSPConfigurer(t.device, t.name, t.bind.ActivityRecorder())
 	err = t.configurer.ConfigureInterface(t.key, t.port)
 	if err != nil {
-		_ = tunIface.Close()
+		if cErr := tunIface.Close(); cErr != nil {
+			log.Debugf("failed to close tun device: %v", cErr)
+		}
 		return nil, fmt.Errorf("error configuring interface: %s", err)
 	}
 

client/iface/netstack/tun.go

@@ -66,7 +66,7 @@ func (t *NetStackTun) Create() (tun.Device, *netstack.Net, error) {
 		}
 	}()
 
-	return nsTunDev, tunNet, nil
+	return t.tundev, tunNet, nil
 }
 
 func (t *NetStackTun) Close() error {

client/internal/engine.go

@@ -543,11 +543,12 @@ func (e *Engine) Start(netbirdConfig *mgmProto.NetbirdConfig, mgmtURL *url.URL)
 	// monitor WireGuard interface lifecycle and restart engine on changes
 	e.wgIfaceMonitor = NewWGIfaceMonitor()
 	e.shutdownWg.Add(1)
+	wgIfaceName := e.wgInterface.Name()
 
 	go func() {
 		defer e.shutdownWg.Done()
 
-		if shouldRestart, err := e.wgIfaceMonitor.Start(e.ctx, e.wgInterface.Name()); shouldRestart {
+		if shouldRestart, err := e.wgIfaceMonitor.Start(e.ctx, wgIfaceName); shouldRestart {
 			log.Infof("WireGuard interface monitor: %s, restarting engine", err)
 			e.triggerClientRestart()
 		} else if err != nil {