Send a TCP RST in response to a TCP SYN-ACK on a listening endpoint

This change better follows what is outlined in RFC 793 section 3.4 figure 12
where a listening socket should not accept a SYN-ACK segment in response to a
(potentially) old SYN segment.

Tests: Test that checks the TCP RST segment sent in response to a TCP SYN-ACK
segment received on a listening TCP endpoint.
PiperOrigin-RevId: 278893114

pkg/tcpip/transport/tcp/accept.go

@@ -25,6 +25,7 @@ import (
 	"gvisor.dev/gvisor/pkg/rand"
 	"gvisor.dev/gvisor/pkg/sleep"
 	"gvisor.dev/gvisor/pkg/tcpip"
+	"gvisor.dev/gvisor/pkg/tcpip/buffer"
 	"gvisor.dev/gvisor/pkg/tcpip/header"
 	"gvisor.dev/gvisor/pkg/tcpip/seqnum"
 	"gvisor.dev/gvisor/pkg/tcpip/stack"
@@ -400,6 +401,14 @@ func (e *endpoint) acceptQueueIsFull() bool {
 // handleListenSegment is called when a listening endpoint receives a segment
 // and needs to handle it.
 func (e *endpoint) handleListenSegment(ctx *listenContext, s *segment) {
+	if s.flagsAreSet(header.TCPFlagSyn | header.TCPFlagAck) {
+		// RFC 793 section 3.4 page 35 (figure 12) outlines that a RST
+		// must be sent in response to a SYN-ACK while in the listen
+		// state to prevent completing a handshake from an old SYN.
+		e.sendTCP(&s.route, s.id, buffer.VectorisedView{}, e.ttl, e.sendTOS, header.TCPFlagRst, s.ackNumber, 0, 0, nil, nil)
+		return
+	}
+
 	// TODO(b/143300739): Use the userMSS of the listening socket
 	// for accepted sockets.
 

pkg/tcpip/transport/tcp/segment.go

@@ -99,8 +99,14 @@ func (s *segment) clone() *segment {
 	return t
 }
 
-func (s *segment) flagIsSet(flag uint8) bool {
-	return (s.flags & flag) != 0
+// flagIsSet checks if at least one flag in flags is set in s.flags.
+func (s *segment) flagIsSet(flags uint8) bool {
+	return s.flags&flags != 0
+}
+
+// flagsAreSet checks if all flags in flags are set in s.flags.
+func (s *segment) flagsAreSet(flags uint8) bool {
+	return s.flags&flags == flags
 }
 
 func (s *segment) decRef() {

pkg/tcpip/transport/tcp/tcp_test.go

@@ -598,6 +598,62 @@ func TestUserSuppliedMSSOnConnectV6(t *testing.T) {
 	}
 }
 
+func TestSendRstOnListenerRxSynAckV4(t *testing.T) {
+	c := context.New(t, defaultMTU)
+	defer c.Cleanup()
+
+	c.Create(-1)
+
+	if err := c.EP.Bind(tcpip.FullAddress{Port: context.StackPort}); err != nil {
+		t.Fatal("Bind failed:", err)
+	}
+
+	if err := c.EP.Listen(10); err != nil {
+		t.Fatal("Listen failed:", err)
+	}
+
+	c.SendPacket(nil, &context.Headers{
+		SrcPort: context.TestPort,
+		DstPort: context.StackPort,
+		Flags:   header.TCPFlagSyn | header.TCPFlagAck,
+		SeqNum:  100,
+		AckNum:  200,
+	})
+
+	checker.IPv4(t, c.GetPacket(), checker.TCP(
+		checker.DstPort(context.TestPort),
+		checker.TCPFlags(header.TCPFlagRst),
+		checker.SeqNum(200)))
+}
+
+func TestSendRstOnListenerRxSynAckV6(t *testing.T) {
+	c := context.New(t, defaultMTU)
+	defer c.Cleanup()
+
+	c.CreateV6Endpoint(true)
+
+	if err := c.EP.Bind(tcpip.FullAddress{Port: context.StackPort}); err != nil {
+		t.Fatal("Bind failed:", err)
+	}
+
+	if err := c.EP.Listen(10); err != nil {
+		t.Fatal("Listen failed:", err)
+	}
+
+	c.SendV6Packet(nil, &context.Headers{
+		SrcPort: context.TestPort,
+		DstPort: context.StackPort,
+		Flags:   header.TCPFlagSyn | header.TCPFlagAck,
+		SeqNum:  100,
+		AckNum:  200,
+	})
+
+	checker.IPv6(t, c.GetV6Packet(), checker.TCP(
+		checker.DstPort(context.TestPort),
+		checker.TCPFlags(header.TCPFlagRst),
+		checker.SeqNum(200)))
+}
+
 func TestTOSV4(t *testing.T) {
 	c := context.New(t, defaultMTU)
 	defer c.Cleanup()