tcpip/link: avoid attempts to deliver packets to nil dispatchers

Fixes #8765

pkg/tcpip/link/channel/channel.go

@@ -192,12 +192,15 @@ func (e *Endpoint) NumQueued() int {
 	return e.q.Num()
 }
 
-// InjectInbound injects an inbound packet.
+// InjectInbound injects an inbound packet. If the endpoint is not attached, the
+// packet is not delivered.
 func (e *Endpoint) InjectInbound(protocol tcpip.NetworkProtocolNumber, pkt stack.PacketBufferPtr) {
 	e.mu.RLock()
 	d := e.dispatcher
 	e.mu.RUnlock()
-	d.DeliverNetworkPacket(protocol, pkt)
+	if d != nil {
+		d.DeliverNetworkPacket(protocol, pkt)
+	}
 }
 
 // Attach saves the stack network-layer dispatcher for use later when packets

pkg/tcpip/link/fdbased/endpoint.go

@@ -785,12 +785,15 @@ func (e *InjectableEndpoint) Attach(dispatcher stack.NetworkDispatcher) {
 	e.dispatcher = dispatcher
 }
 
-// InjectInbound injects an inbound packet.
+// InjectInbound injects an inbound packet. If the endpoint is not attached, the
+// packet is not delivered.
 func (e *InjectableEndpoint) InjectInbound(protocol tcpip.NetworkProtocolNumber, pkt stack.PacketBufferPtr) {
 	e.mu.RLock()
 	d := e.dispatcher
 	e.mu.RUnlock()
-	d.DeliverNetworkPacket(protocol, pkt)
+	if d != nil {
+		d.DeliverNetworkPacket(protocol, pkt)
+	}
 }
 
 // NewInjectable creates a new fd-based InjectableEndpoint.

pkg/tcpip/link/loopback/loopback.go

@@ -81,7 +81,8 @@ func (*endpoint) LinkAddress() tcpip.LinkAddress {
 // Wait implements stack.LinkEndpoint.Wait.
 func (*endpoint) Wait() {}
 
-// WritePackets implements stack.LinkEndpoint.WritePackets.
+// WritePackets implements stack.LinkEndpoint.WritePackets. If the endpoint is
+// not attached, the packets are not delivered.
 func (e *endpoint) WritePackets(pkts stack.PacketBufferList) (int, tcpip.Error) {
 	e.mu.RLock()
 	d := e.dispatcher
@@ -93,7 +94,9 @@ func (e *endpoint) WritePackets(pkts stack.PacketBufferList) (int, tcpip.Error)
 		newPkt := stack.NewPacketBuffer(stack.PacketBufferOptions{
 			Payload: pkt.ToBuffer(),
 		})
-		d.DeliverNetworkPacket(pkt.NetworkProtocolNumber, newPkt)
+		if d != nil {
+			d.DeliverNetworkPacket(pkt.NetworkProtocolNumber, newPkt)
+		}
 		newPkt.DecRef()
 	}
 	return pkts.Len(), nil

pkg/tcpip/link/waitable/waitable.go

@@ -63,7 +63,9 @@ func (e *Endpoint) DeliverNetworkPacket(protocol tcpip.NetworkProtocolNumber, pk
 	e.mu.RLock()
 	d := e.dispatcher
 	e.mu.RUnlock()
-	d.DeliverNetworkPacket(protocol, pkt)
+	if d != nil {
+		d.DeliverNetworkPacket(protocol, pkt)
+	}
 	e.dispatchGate.Leave()
 }
 
@@ -75,7 +77,9 @@ func (e *Endpoint) DeliverLinkPacket(protocol tcpip.NetworkProtocolNumber, pkt s
 	e.mu.RLock()
 	d := e.dispatcher
 	e.mu.RUnlock()
-	d.DeliverLinkPacket(protocol, pkt)
+	if d != nil {
+		d.DeliverLinkPacket(protocol, pkt)
+	}
 	e.dispatchGate.Leave()
 }