Do not reject TCP SYN w/ ECN flags set.

This change does not add support for ECN flags in Netstack. It just ensures we
don't reject valid SYN packets with ECN bits set.

For a complete ECN implementation we would need to implement the following
section https://datatracker.ietf.org/doc/html/rfc3168#section-6 as well as the
IP bits to set the ECN bits accordingly.

Fixes #7075

PiperOrigin-RevId: 423345617

pkg/tcpip/header/tcp.go

@@ -60,7 +60,7 @@ func (f TCPFlags) Contains(o TCPFlags) bool {
 
 // String implements Stringer.String.
 func (f TCPFlags) String() string {
-	flagsStr := []byte("FSRPAU")
+	flagsStr := []byte("FSRPAUEC")
 	for i := range flagsStr {
 		if f&(1<<uint(i)) == 0 {
 			flagsStr[i] = ' '
@@ -77,6 +77,8 @@ const (
 	TCPFlagPsh
 	TCPFlagAck
 	TCPFlagUrg
+	TCPFlagEce
+	TCPFlagCwr
 )
 
 // Options that may be present in a TCP segment.
@@ -152,6 +154,10 @@ type TCPSynOptions struct {
 
 	// SACKPermitted is true if the SACK option was provided in the SYN/SYN-ACK.
 	SACKPermitted bool
+
+	// Flags if specified are set on the outgoing SYN. The SYN flag is
+	// always set.
+	Flags TCPFlags
 }
 
 // SACKBlock represents a single contiguous SACK block.

pkg/tcpip/header/tcp_test.go

@@ -152,14 +152,16 @@ func TestTCPFlags(t *testing.T) {
 		flags header.TCPFlags
 		want  string
 	}{
-		{header.TCPFlagFin, "F     "},
-		{header.TCPFlagSyn, " S    "},
-		{header.TCPFlagRst, "  R   "},
-		{header.TCPFlagPsh, "   P  "},
-		{header.TCPFlagAck, "    A "},
-		{header.TCPFlagUrg, "     U"},
-		{header.TCPFlagSyn | header.TCPFlagAck, " S  A "},
-		{header.TCPFlagFin | header.TCPFlagAck, "F   A "},
+		{header.TCPFlagFin, "F       "},
+		{header.TCPFlagSyn, " S      "},
+		{header.TCPFlagRst, "  R     "},
+		{header.TCPFlagPsh, "   P    "},
+		{header.TCPFlagAck, "    A   "},
+		{header.TCPFlagUrg, "     U  "},
+		{header.TCPFlagEce, "      E "},
+		{header.TCPFlagCwr, "       C"},
+		{header.TCPFlagSyn | header.TCPFlagAck, " S  A   "},
+		{header.TCPFlagFin | header.TCPFlagAck, "F   A   "},
 	} {
 		if got := tt.flags.String(); got != tt.want {
 			t.Errorf("got TCPFlags(%#b).String() = %s, want = %s", tt.flags, got, tt.want)

pkg/tcpip/transport/tcp/BUILD

@@ -83,6 +83,7 @@ go_test(
     size = "large",
     srcs = [
         "dual_stack_test.go",
+        "forwarder_test.go",
         "rcv_test.go",
         "sack_scoreboard_test.go",
         "tcp_noracedetector_test.go",

pkg/tcpip/transport/tcp/accept.go

@@ -443,7 +443,7 @@ func (e *endpoint) handleListenSegment(ctx *listenContext, s *segment) tcpip.Err
 		e.stack.Stats().DroppedPackets.Increment()
 		return nil
 
-	case s.flags == header.TCPFlagSyn:
+	case s.flags.Contains(header.TCPFlagSyn):
 		if e.acceptQueueIsFull() {
 			e.stack.Stats().TCP.ListenOverflowSynDrop.Increment()
 			e.stats.ReceiveErrors.ListenOverflowSynDrop.Increment()

pkg/tcpip/transport/tcp/forwarder.go

@@ -68,8 +68,8 @@ func (f *Forwarder) HandlePacket(id stack.TransportEndpointID, pkt *stack.Packet
 	s := newIncomingSegment(id, f.stack.Clock(), pkt)
 	defer s.decRef()
 
-	// We only care about well-formed SYN packets.
-	if !s.parse(pkt.RXTransportChecksumValidated) || !s.csumValid || s.flags != header.TCPFlagSyn {
+	// We only care about well-formed SYN packets (not SYN-ACK) packets.
+	if !s.parse(pkt.RXTransportChecksumValidated) || !s.csumValid || !s.flags.Contains(header.TCPFlagSyn) || s.flags.Contains(header.TCPFlagAck) {
 		return false
 	}
 

pkg/tcpip/transport/tcp/forwarder_test.go

@@ -0,0 +1,101 @@
+// Copyright 2018 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package tcp_test
+
+import (
+	"testing"
+	"time"
+
+	"gvisor.dev/gvisor/pkg/tcpip"
+	"gvisor.dev/gvisor/pkg/tcpip/header"
+	"gvisor.dev/gvisor/pkg/tcpip/transport/tcp"
+	"gvisor.dev/gvisor/pkg/tcpip/transport/tcp/testing/context"
+)
+
+func TestForwarderSendMSSLessThanMTU(t *testing.T) {
+	const maxPayload = 100
+	const mtu = 1200
+	c := context.New(t, mtu)
+	defer c.Cleanup()
+
+	s := c.Stack()
+	ch := make(chan tcpip.Error, 1)
+	f := tcp.NewForwarder(s, 65536, 10, func(r *tcp.ForwarderRequest) {
+		var err tcpip.Error
+		c.EP, err = r.CreateEndpoint(&c.WQ)
+		ch <- err
+		close(ch)
+	})
+	s.SetTransportProtocolHandler(tcp.ProtocolNumber, f.HandlePacket)
+
+	// Do 3-way handshake.
+	c.PassiveConnect(maxPayload, -1, header.TCPSynOptions{MSS: mtu - header.IPv4MinimumSize - header.TCPMinimumSize})
+
+	// Wait for connection to be available.
+	select {
+	case err := <-ch:
+		if err != nil {
+			t.Fatalf("Error creating endpoint: %s", err)
+		}
+	case <-time.After(2 * time.Second):
+		t.Fatalf("Timed out waiting for connection")
+	}
+
+	// Check that data gets properly segmented.
+	testBrokenUpWrite(t, c, maxPayload)
+}
+
+func TestForwarderDoesNotRejectECNFlags(t *testing.T) {
+	testCases := []struct {
+		name  string
+		flags header.TCPFlags
+	}{
+		{name: "non-setup ECN SYN w/ ECE", flags: header.TCPFlagEce},
+		{name: "non-setup ECN SYN w/ CWR", flags: header.TCPFlagCwr},
+		{name: "setup ECN SYN", flags: header.TCPFlagEce | header.TCPFlagCwr},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			const maxPayload = 100
+			const mtu = 1200
+			c := context.New(t, mtu)
+			defer c.Cleanup()
+
+			s := c.Stack()
+			ch := make(chan tcpip.Error, 1)
+			f := tcp.NewForwarder(s, 65536, 10, func(r *tcp.ForwarderRequest) {
+				var err tcpip.Error
+				c.EP, err = r.CreateEndpoint(&c.WQ)
+				ch <- err
+				close(ch)
+			})
+			s.SetTransportProtocolHandler(tcp.ProtocolNumber, f.HandlePacket)
+
+			// Do 3-way handshake.
+			c.PassiveConnect(maxPayload, -1, header.TCPSynOptions{MSS: mtu - header.IPv4MinimumSize - header.TCPMinimumSize, Flags: tc.flags})
+
+			// Wait for connection to be available.
+			select {
+			case err := <-ch:
+				if err != nil {
+					t.Fatalf("Error creating endpoint: %s", err)
+				}
+			case <-time.After(2 * time.Second):
+				t.Fatalf("Timed out waiting for connection")
+			}
+		})
+	}
+}

pkg/tcpip/transport/tcp/tcp_test.go

@@ -3631,38 +3631,6 @@ func TestSynCookiePassiveSendMSSLessThanMTU(t *testing.T) {
 	testBrokenUpWrite(t, c, maxPayload)
 }
 
-func TestForwarderSendMSSLessThanMTU(t *testing.T) {
-	const maxPayload = 100
-	const mtu = 1200
-	c := context.New(t, mtu)
-	defer c.Cleanup()
-
-	s := c.Stack()
-	ch := make(chan tcpip.Error, 1)
-	f := tcp.NewForwarder(s, 65536, 10, func(r *tcp.ForwarderRequest) {
-		var err tcpip.Error
-		c.EP, err = r.CreateEndpoint(&c.WQ)
-		ch <- err
-	})
-	s.SetTransportProtocolHandler(tcp.ProtocolNumber, f.HandlePacket)
-
-	// Do 3-way handshake.
-	c.PassiveConnect(maxPayload, -1, header.TCPSynOptions{MSS: mtu - header.IPv4MinimumSize - header.TCPMinimumSize})
-
-	// Wait for connection to be available.
-	select {
-	case err := <-ch:
-		if err != nil {
-			t.Fatalf("Error creating endpoint: %s", err)
-		}
-	case <-time.After(2 * time.Second):
-		t.Fatalf("Timed out waiting for connection")
-	}
-
-	// Check that data gets properly segmented.
-	testBrokenUpWrite(t, c, maxPayload)
-}
-
 func TestSynOptionsOnActiveConnect(t *testing.T) {
 	const mtu = 1400
 	c := context.New(t, mtu)
@@ -8671,3 +8639,67 @@ func TestTimestampSynCookies(t *testing.T) {
 		t.Fatalf("got TSVal = %d, want %d", got, want)
 	}
 }
+
+// TestECNFlagsAccept tests that an ECN non-setup/setup SYN is accepted
+// and the connection is correctly completed.
+func TestECNFlagsAccept(t *testing.T) {
+	testCases := []struct {
+		name  string
+		flags header.TCPFlags
+	}{
+		{name: "non-setup ECN SYN w/ ECE", flags: header.TCPFlagEce},
+		{name: "non-setup ECN SYN w/ CWR", flags: header.TCPFlagCwr},
+		{name: "setup ECN SYN", flags: header.TCPFlagEce | header.TCPFlagCwr},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+
+			c := context.New(t, defaultMTU)
+			defer c.Cleanup()
+
+			// Create EP and start listening.
+			wq := &waiter.Queue{}
+			ep, err := c.Stack().NewEndpoint(tcp.ProtocolNumber, ipv4.ProtocolNumber, wq)
+			if err != nil {
+				t.Fatalf("NewEndpoint failed: %s", err)
+			}
+			defer ep.Close()
+
+			if err := ep.Bind(tcpip.FullAddress{Port: context.StackPort}); err != nil {
+				t.Fatalf("Bind failed: %s", err)
+			}
+
+			if err := ep.Listen(10); err != nil {
+				t.Fatalf("Listen failed: %s", err)
+			}
+
+			// Do 3-way handshake.
+			const maxPayload = 100
+
+			c.PassiveConnect(maxPayload, -1 /* wndScale */, header.TCPSynOptions{MSS: defaultIPv4MSS, Flags: tc.flags})
+
+			// Try to accept the connection.
+			we, ch := waiter.NewChannelEntry(waiter.ReadableEvents)
+			wq.EventRegister(&we)
+			defer wq.EventUnregister(&we)
+
+			c.EP, _, err = ep.Accept(nil)
+			if cmp.Equal(&tcpip.ErrWouldBlock{}, err) {
+				// Wait for connection to be established.
+				select {
+				case <-ch:
+					c.EP, _, err = ep.Accept(nil)
+					if err != nil {
+						t.Fatalf("Accept failed: %s", err)
+					}
+
+				case <-time.After(1 * time.Second):
+					t.Fatalf("Timed out waiting for accept")
+				}
+			} else if err != nil {
+				t.Fatalf("Accept failed: %s", err)
+			}
+		})
+	}
+}

pkg/tcpip/transport/tcp/testing/context/context.go

@@ -1158,7 +1158,7 @@ func (c *Context) PassiveConnectWithOptions(maxPayload, wndScale int, synOptions
 	c.SendPacket(nil, &Headers{
 		SrcPort: TestPort,
 		DstPort: StackPort,
-		Flags:   header.TCPFlagSyn,
+		Flags:   header.TCPFlagSyn | synOptions.Flags,
 		SeqNum:  iss,
 		RcvWnd:  30000,
 		TCPOpts: opts[:offset],

test/packetimpact/testbench/layers_test.go

@@ -189,7 +189,7 @@ func TestLayerStringFormat(t *testing.T) {
 				"SeqNum:3452155723 " +
 				"AckNum:2596996163 " +
 				"DataOffset:5 " +
-				"Flags:  R A  " +
+				"Flags:  R A    " +
 				"WindowSize:64240 " +
 				"Checksum:11819" +
 				"}",