Explicitly bind endpoint to a NIC

Previously, any time a datagram-based network endpoint (e.g. UDP) was
bound, the bound NIC is always set based on the bound address (if
specified). However, we should only consider the endpoint bound to
an NIC if a NIC was explicitly bound to.

If an endpoint has been bound to an address and attempts to send packets
to an unconnected remote, the endpoint will default to sending packets
through the bound address' NIC if not explicitly bound to a NIC.

Updates #6565.

PiperOrigin-RevId: 396712415

4 files changed, 122 insertions, 16 deletions

diff --git a/pkg/tcpip/transport/internal/network/BUILD b/pkg/tcpip/transport/internal/network/BUILD
index d10e3f13a..d6d3f52a3 100644
--- a/pkg/tcpip/transport/internal/network/BUILD
+++ b/pkg/tcpip/transport/internal/network/BUILD
@@ -32,6 +32,7 @@ go_test(
         "//pkg/tcpip/faketime",
         "//pkg/tcpip/header",
         "//pkg/tcpip/link/channel",
+        "//pkg/tcpip/link/loopback",
         "//pkg/tcpip/network/ipv4",
         "//pkg/tcpip/network/ipv6",
         "//pkg/tcpip/stack",
diff --git a/pkg/tcpip/transport/internal/network/endpoint.go b/pkg/tcpip/transport/internal/network/endpoint.go
index c5b575e1c..09b629022 100644
--- a/pkg/tcpip/transport/internal/network/endpoint.go
+++ b/pkg/tcpip/transport/internal/network/endpoint.go
@@ -44,8 +44,9 @@ type Endpoint struct {
 	state uint32
 
 	// The following fields are protected by mu.
-	mu   sync.RWMutex `state:"nosave"`
-	info stack.TransportEndpointInfo
+	mu       sync.RWMutex `state:"nosave"`
+	wasBound bool
+	info     stack.TransportEndpointInfo
 	// owner is the owner of transmitted packets.
 	owner                tcpip.PacketOwner
 	writeShutdown        bool
@@ -248,6 +249,9 @@ func (e *Endpoint) AcquireContextForWrite(opts tcpip.WriteOptions) (WriteContext
 
 			nicID = e.info.BindNICID
 		}
+		if nicID == 0 {
+			nicID = e.info.RegisterNICID
+		}
 
 		dst, netProto, err := e.checkV4MappedLocked(*opts.To)
 		if err != nil {
@@ -294,9 +298,9 @@ func (e *Endpoint) Disconnect() {
 	}
 
 	// Exclude ephemerally bound endpoints.
-	if e.info.BindNICID != 0 || e.info.ID.LocalAddress == "" {
+	if e.wasBound {
 		e.info.ID = stack.TransportEndpointID{
-			LocalAddress: e.info.ID.LocalAddress,
+			LocalAddress: e.info.BindAddr,
 		}
 		e.setEndpointState(transport.DatagramEndpointStateBound)
 	} else {
@@ -477,10 +481,12 @@ func (e *Endpoint) BindAndThen(addr tcpip.FullAddress, f func(tcpip.NetworkProto
 		return err
 	}
 
+	e.wasBound = true
+
 	e.info.ID = stack.TransportEndpointID{
 		LocalAddress: addr.Addr,
 	}
-	e.info.BindNICID = nicID
+	e.info.BindNICID = addr.NIC
 	e.info.RegisterNICID = nicID
 	e.info.BindAddr = addr.Addr
 	e.effectiveNetProto = netProto
@@ -488,6 +494,13 @@ func (e *Endpoint) BindAndThen(addr tcpip.FullAddress, f func(tcpip.NetworkProto
 	return nil
 }
 
+// WasBound returns true iff the endpoint was ever bound.
+func (e *Endpoint) WasBound() bool {
+	e.mu.RLock()
+	defer e.mu.RUnlock()
+	return e.wasBound
+}
+
 // GetLocalAddress returns the address that the endpoint is bound to.
 func (e *Endpoint) GetLocalAddress() tcpip.FullAddress {
 	e.mu.RLock()
diff --git a/pkg/tcpip/transport/internal/network/endpoint_test.go b/pkg/tcpip/transport/internal/network/endpoint_test.go
index 2c43eb66a..d99c961c3 100644
--- a/pkg/tcpip/transport/internal/network/endpoint_test.go
+++ b/pkg/tcpip/transport/internal/network/endpoint_test.go
@@ -15,6 +15,7 @@
 package network_test
 
 import (
+	"fmt"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
@@ -24,6 +25,7 @@ import (
 	"gvisor.dev/gvisor/pkg/tcpip/faketime"
 	"gvisor.dev/gvisor/pkg/tcpip/header"
 	"gvisor.dev/gvisor/pkg/tcpip/link/channel"
+	"gvisor.dev/gvisor/pkg/tcpip/link/loopback"
 	"gvisor.dev/gvisor/pkg/tcpip/network/ipv4"
 	"gvisor.dev/gvisor/pkg/tcpip/network/ipv6"
 	"gvisor.dev/gvisor/pkg/tcpip/stack"
@@ -33,17 +35,15 @@ import (
 	"gvisor.dev/gvisor/pkg/tcpip/transport/udp"
 )
 
-func TestEndpointStateTransitions(t *testing.T) {
-	const (
-		nicID = 1
-	)
+var (
+	ipv4NICAddr    = testutil.MustParse4("1.2.3.4")
+	ipv6NICAddr    = testutil.MustParse6("a::1")
+	ipv4RemoteAddr = testutil.MustParse4("6.7.8.9")
+	ipv6RemoteAddr = testutil.MustParse6("b::1")
+)
 
-	var (
-		ipv4NICAddr    = testutil.MustParse4("1.2.3.4")
-		ipv6NICAddr    = testutil.MustParse6("a::1")
-		ipv4RemoteAddr = testutil.MustParse4("6.7.8.9")
-		ipv6RemoteAddr = testutil.MustParse6("b::1")
-	)
+func TestEndpointStateTransitions(t *testing.T) {
+	const nicID = 1
 
 	data := buffer.View([]byte{1, 2, 4, 5})
 	v4Checker := func(t *testing.T, b buffer.View) {
@@ -139,6 +139,7 @@ func TestEndpointStateTransitions(t *testing.T) {
 			var ops tcpip.SocketOptions
 			var ep network.Endpoint
 			ep.Init(s, test.netProto, udp.ProtocolNumber, &ops)
+			defer ep.Close()
 			if state := ep.State(); state != transport.DatagramEndpointStateInitial {
 				t.Fatalf("got ep.State() = %s, want = %s", state, transport.DatagramEndpointStateInitial)
 			}
@@ -207,3 +208,94 @@ func TestEndpointStateTransitions(t *testing.T) {
 		})
 	}
 }
+
+func TestBindNICID(t *testing.T) {
+	const nicID = 1
+
+	tests := []struct {
+		name     string
+		netProto tcpip.NetworkProtocolNumber
+		bindAddr tcpip.Address
+		unicast  bool
+	}{
+		{
+			name:     "IPv4 multicast",
+			netProto: ipv4.ProtocolNumber,
+			bindAddr: header.IPv4AllSystems,
+			unicast:  false,
+		},
+		{
+			name:     "IPv6 multicast",
+			netProto: ipv6.ProtocolNumber,
+			bindAddr: header.IPv6AllNodesMulticastAddress,
+			unicast:  false,
+		},
+		{
+			name:     "IPv4 unicast",
+			netProto: ipv4.ProtocolNumber,
+			bindAddr: ipv4NICAddr,
+			unicast:  true,
+		},
+		{
+			name:     "IPv6 unicast",
+			netProto: ipv6.ProtocolNumber,
+			bindAddr: ipv6NICAddr,
+			unicast:  true,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			for _, testBindNICID := range []tcpip.NICID{0, nicID} {
+				t.Run(fmt.Sprintf("BindNICID=%d", testBindNICID), func(t *testing.T) {
+					s := stack.New(stack.Options{
+						NetworkProtocols:   []stack.NetworkProtocolFactory{ipv4.NewProtocol, ipv6.NewProtocol},
+						TransportProtocols: []stack.TransportProtocolFactory{udp.NewProtocol},
+						Clock:              &faketime.NullClock{},
+					})
+					if err := s.CreateNIC(nicID, loopback.New()); err != nil {
+						t.Fatalf("s.CreateNIC(%d, _): %s", nicID, err)
+					}
+
+					if err := s.AddAddress(nicID, ipv4.ProtocolNumber, ipv4NICAddr); err != nil {
+						t.Fatalf("s.AddAddress(%d, %d, %s): %s", nicID, ipv4.ProtocolNumber, ipv4NICAddr, err)
+					}
+					if err := s.AddAddress(nicID, ipv6.ProtocolNumber, ipv6NICAddr); err != nil {
+						t.Fatalf("s.AddAddress(%d, %d, %s): %s", nicID, ipv6.ProtocolNumber, ipv6NICAddr, err)
+					}
+
+					var ops tcpip.SocketOptions
+					var ep network.Endpoint
+					ep.Init(s, test.netProto, udp.ProtocolNumber, &ops)
+					defer ep.Close()
+					if ep.WasBound() {
+						t.Fatal("got ep.WasBound() = true, want = false")
+					}
+					wantInfo := stack.TransportEndpointInfo{NetProto: test.netProto, TransProto: udp.ProtocolNumber}
+					if diff := cmp.Diff(wantInfo, ep.Info()); diff != "" {
+						t.Fatalf("ep.Info() mismatch (-want +got):\n%s", diff)
+					}
+
+					bindAddr := tcpip.FullAddress{Addr: test.bindAddr, NIC: testBindNICID}
+					if err := ep.Bind(bindAddr); err != nil {
+						t.Fatalf("ep.Bind(%#v): %s", bindAddr, err)
+					}
+					if !ep.WasBound() {
+						t.Error("got ep.WasBound() = false, want = true")
+					}
+					wantInfo.ID = stack.TransportEndpointID{LocalAddress: bindAddr.Addr}
+					wantInfo.BindAddr = bindAddr.Addr
+					wantInfo.BindNICID = bindAddr.NIC
+					if test.unicast {
+						wantInfo.RegisterNICID = nicID
+					} else {
+						wantInfo.RegisterNICID = bindAddr.NIC
+					}
+					if diff := cmp.Diff(wantInfo, ep.Info()); diff != "" {
+						t.Errorf("ep.Info() mismatch (-want +got):\n%s", diff)
+					}
+				})
+			}
+		})
+	}
+}
diff --git a/pkg/tcpip/transport/udp/endpoint.go b/pkg/tcpip/transport/udp/endpoint.go
index f171a16f8..4255457f9 100644
--- a/pkg/tcpip/transport/udp/endpoint.go
+++ b/pkg/tcpip/transport/udp/endpoint.go
@@ -547,7 +547,7 @@ func (e *endpoint) Disconnect() tcpip.Error {
 	info := e.net.Info()
 	info.ID.LocalPort = e.localPort
 	info.ID.RemotePort = e.remotePort
-	if info.BindNICID != 0 || info.ID.LocalAddress == "" {
+	if e.net.WasBound() {
 		var err tcpip.Error
 		id = stack.TransportEndpointID{
 			LocalPort:    info.ID.LocalPort,