Bind loopback subnets' lifetime to perm address

The lifetime of addreses in a loopback interface's associated subnets
should be bound to their respective permanent addresses.

This change also fixes a race when the stack attempts to get an IPv4
rereferencedNetworkEndpoint for an address in an associated subnet on
a loopback interface. Before this change, the stack would only check
if an IPv4 address is contained in an associated subnet while holding
a read lock but wouldn't do this same check after releasing the read
lock for a write lock to create a temporary address. This may cause
the stack to bind the lifetime of the address to a new (temporary)
endpoint instead of the associated subnet's permanent address.

Test: integration_test.TestLoopbackSubnetLifetimeBoundToAddr
PiperOrigin-RevId: 332094719

pkg/tcpip/stack/nic.go

@@ -665,33 +665,15 @@ func (n *NIC) getRefOrCreateTemp(protocol tcpip.NetworkProtocolNumber, address t
 		}
 	}
 
-	// Check if address is a broadcast address for the endpoint's network.
-	//
-	// Only IPv4 has a notion of broadcast addresses.
 	if protocol == header.IPv4ProtocolNumber {
-		if ref := n.getRefForBroadcastRLocked(address); ref != nil {
+		if ref := n.getIPv4RefForBroadcastOrLoopbackRLocked(address); ref != nil {
 			n.mu.RUnlock()
 			return ref
 		}
 	}
-
-	// A usable reference was not found, create a temporary one if requested by
-	// the caller or if the IPv4 address is found in the NIC's subnets and the NIC
-	// is a loopback interface.
-	createTempEP := spoofingOrPromiscuous
-	if !createTempEP && n.isLoopback() && protocol == header.IPv4ProtocolNumber {
-		for _, r := range n.mu.endpoints {
-			addr := r.addrWithPrefix()
-			subnet := addr.Subnet()
-			if subnet.Contains(address) {
-				createTempEP = true
-				break
-			}
-		}
-	}
 	n.mu.RUnlock()
 
-	if !createTempEP {
+	if !spoofingOrPromiscuous {
 		return nil
 	}
 
@@ -704,20 +686,21 @@ func (n *NIC) getRefOrCreateTemp(protocol tcpip.NetworkProtocolNumber, address t
 	return ref
 }
 
-// getRefForBroadcastLocked returns an endpoint where address is the IPv4
-// broadcast address for the endpoint's network.
+// getRefForBroadcastOrLoopbackRLocked returns an endpoint whose address is the
+// broadcast address for the endpoint's network or an address in the endpoint's
+// subnet if the NIC is a loopback interface. This matches linux behaviour.
 //
-// n.mu MUST be read locked.
-func (n *NIC) getRefForBroadcastRLocked(address tcpip.Address) *referencedNetworkEndpoint {
+// n.mu MUST be read or write locked.
+func (n *NIC) getIPv4RefForBroadcastOrLoopbackRLocked(address tcpip.Address) *referencedNetworkEndpoint {
 	for _, ref := range n.mu.endpoints {
-		// Only IPv4 has a notion of broadcast addresses.
+		// Only IPv4 has a notion of broadcast addresses or considers the loopback
+		// interface bound to an address's whole subnet (on linux).
 		if ref.protocol != header.IPv4ProtocolNumber {
 			continue
 		}
 
-		addr := ref.addrWithPrefix()
-		subnet := addr.Subnet()
-		if subnet.IsBroadcast(address) && ref.tryIncRef() {
+		subnet := ref.addrWithPrefix().Subnet()
+		if (subnet.IsBroadcast(address) || (n.isLoopback() && subnet.Contains(address))) && ref.isValidForOutgoingRLocked() && ref.tryIncRef() {
 			return ref
 		}
 	}
@@ -745,11 +728,8 @@ func (n *NIC) getRefOrCreateTempLocked(protocol tcpip.NetworkProtocolNumber, add
 		n.removeEndpointLocked(ref)
 	}
 
-	// Check if address is a broadcast address for an endpoint's network.
-	//
-	// Only IPv4 has a notion of broadcast addresses.
 	if protocol == header.IPv4ProtocolNumber {
-		if ref := n.getRefForBroadcastRLocked(address); ref != nil {
+		if ref := n.getIPv4RefForBroadcastOrLoopbackRLocked(address); ref != nil {
 			return ref
 		}
 	}

pkg/tcpip/tests/integration/loopback_test.go

@@ -187,3 +187,64 @@ func TestLoopbackAcceptAllInSubnet(t *testing.T) {
 		})
 	}
 }
+
+// TestLoopbackSubnetLifetimeBoundToAddr tests that the lifetime of an address
+// in a loopback interface's associated subnet is bound to the permanently bound
+// address.
+func TestLoopbackSubnetLifetimeBoundToAddr(t *testing.T) {
+	const nicID = 1
+
+	protoAddr := tcpip.ProtocolAddress{
+		Protocol:          ipv4.ProtocolNumber,
+		AddressWithPrefix: ipv4Addr,
+	}
+	addrBytes := []byte(ipv4Addr.Address)
+	addrBytes[len(addrBytes)-1]++
+	otherAddr := tcpip.Address(addrBytes)
+
+	s := stack.New(stack.Options{
+		NetworkProtocols: []stack.NetworkProtocol{ipv4.NewProtocol()},
+	})
+	if err := s.CreateNIC(nicID, loopback.New()); err != nil {
+		t.Fatalf("s.CreateNIC(%d, _): %s", nicID, err)
+	}
+	if err := s.AddProtocolAddress(nicID, protoAddr); err != nil {
+		t.Fatalf("s.AddProtocolAddress(%d, %#v): %s", nicID, protoAddr, err)
+	}
+	s.SetRouteTable([]tcpip.Route{
+		tcpip.Route{
+			Destination: header.IPv4EmptySubnet,
+			NIC:         nicID,
+		},
+	})
+
+	r, err := s.FindRoute(nicID, otherAddr, remoteIPv4Addr, ipv4.ProtocolNumber, false /* multicastLoop */)
+	if err != nil {
+		t.Fatalf("s.FindRoute(%d, %s, %s, %d, false): %s", nicID, otherAddr, remoteIPv4Addr, ipv4.ProtocolNumber, err)
+	}
+	defer r.Release()
+
+	params := stack.NetworkHeaderParams{
+		Protocol: 111,
+		TTL:      64,
+		TOS:      stack.DefaultTOS,
+	}
+	data := buffer.View([]byte{1, 2, 3, 4})
+	if err := r.WritePacket(nil /* gso */, params, stack.NewPacketBuffer(stack.PacketBufferOptions{
+		ReserveHeaderBytes: int(r.MaxHeaderLength()),
+		Data:               data.ToVectorisedView(),
+	})); err != nil {
+		t.Fatalf("r.WritePacket(nil, %#v, _): %s", params, err)
+	}
+
+	// Removing the address should make the endpoint invalid.
+	if err := s.RemoveAddress(nicID, protoAddr.AddressWithPrefix.Address); err != nil {
+		t.Fatalf("s.RemoveAddress(%d, %s): %s", nicID, protoAddr.AddressWithPrefix.Address, err)
+	}
+	if err := r.WritePacket(nil /* gso */, params, stack.NewPacketBuffer(stack.PacketBufferOptions{
+		ReserveHeaderBytes: int(r.MaxHeaderLength()),
+		Data:               data.ToVectorisedView(),
+	})); err != tcpip.ErrInvalidEndpointState {
+		t.Fatalf("got r.WritePacket(nil, %#v, _) = %s, want = %s", params, err, tcpip.ErrInvalidEndpointState)
+	}
+}