tun: use correct IP header comparisons in tcpGRO() and tcpPacketsCanCoalesce()

tcpGRO() was using an incorrect IPv4 more fragments bit mask.

tcpPacketsCanCoalesce() was not distinguishing tcp6 from tcp4, and TTL
values were not compared. TTL values should be equal at the IP layer,
otherwise the packets should not coalesce. This tracks with the kernel.

Reviewed-by: Denton Gentry <dgentry@tailscale.com>
Signed-off-by: Jordan Whited <jordan@tailscale.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

tun/tcp_offload_linux.go

@@ -189,14 +189,29 @@ func tcpPacketsCanCoalesce(pkt []byte, iphLen, tcphLen uint8, seq uint32, pshSet
 			return coalesceUnavailable
 		}
 	}
-	if pkt[1] != pktTarget[1] {
+	if pkt[0]>>4 == 6 {
-		// cannot coalesce with unequal ToS values
+		if pkt[0] != pktTarget[0] || pkt[1]>>4 != pktTarget[1]>>4 {
-		return coalesceUnavailable
+			// cannot coalesce with unequal Traffic class values
-	}
+			return coalesceUnavailable
-	if pkt[6]>>5 != pktTarget[6]>>5 {
+		}
-		// cannot coalesce with unequal DF or reserved bits. MF is checked
+		if pkt[7] != pktTarget[7] {
-		// further up the stack.
+			// cannot coalesce with unequal Hop limit values
-		return coalesceUnavailable
+			return coalesceUnavailable
+		}
+	} else {
+		if pkt[1] != pktTarget[1] {
+			// cannot coalesce with unequal ToS values
+			return coalesceUnavailable
+		}
+		if pkt[6]>>5 != pktTarget[6]>>5 {
+			// cannot coalesce with unequal DF or reserved bits. MF is checked
+			// further up the stack.
+			return coalesceUnavailable
+		}
+		if pkt[8] != pktTarget[8] {
+			// cannot coalesce with unequal TTL values
+			return coalesceUnavailable
+		}
 	}
 	// seq adjacency
 	lhsLen := item.gsoSize
@@ -366,7 +381,7 @@ func coalesceTCPPackets(mode canCoalesce, pkt []byte, pktBuffsIndex int, gsoSize
 }
 
 const (
-	ipv4FlagMoreFragments = 0x80
+	ipv4FlagMoreFragments uint8 = 0x20
 )
 
 const (
@@ -409,7 +424,7 @@ func tcpGRO(bufs [][]byte, offset int, pktI int, table *tcpGROTable, isV6 bool)
 		return false
 	}
 	if !isV6 {
-		if pkt[6]&ipv4FlagMoreFragments != 0 || (pkt[6]<<3 != 0 || pkt[7] != 0) {
+		if pkt[6]&ipv4FlagMoreFragments != 0 || pkt[6]<<3 != 0 || pkt[7] != 0 {
 			// no GRO support for fragmented segments for now
 			return false
 		}

tun/tcp_offload_linux_test.go

@@ -28,19 +28,23 @@ var (
 	ip6PortC = netip.MustParseAddrPort("[2001:db8::3]:1")
 )
 
-func tcp4Packet(srcIPPort, dstIPPort netip.AddrPort, flags header.TCPFlags, segmentSize, seq uint32) []byte {
+func tcp4PacketMutateIPFields(srcIPPort, dstIPPort netip.AddrPort, flags header.TCPFlags, segmentSize, seq uint32, ipFn func(*header.IPv4Fields)) []byte {
 	totalLen := 40 + segmentSize
 	b := make([]byte, offset+int(totalLen), 65535)
 	ipv4H := header.IPv4(b[offset:])
 	srcAs4 := srcIPPort.Addr().As4()
 	dstAs4 := dstIPPort.Addr().As4()
-	ipv4H.Encode(&header.IPv4Fields{
+	ipFields := &header.IPv4Fields{
 		SrcAddr:     tcpip.Address(srcAs4[:]),
 		DstAddr:     tcpip.Address(dstAs4[:]),
 		Protocol:    unix.IPPROTO_TCP,
 		TTL:         64,
 		TotalLength: uint16(totalLen),
-	})
+	}
+	if ipFn != nil {
+		ipFn(ipFields)
+	}
+	ipv4H.Encode(ipFields)
 	tcpH := header.TCP(b[offset+20:])
 	tcpH.Encode(&header.TCPFields{
 		SrcPort:    srcIPPort.Port(),
@@ -57,19 +61,27 @@ func tcp4Packet(srcIPPort, dstIPPort netip.AddrPort, flags header.TCPFlags, segm
 	return b
 }
 
-func tcp6Packet(srcIPPort, dstIPPort netip.AddrPort, flags header.TCPFlags, segmentSize, seq uint32) []byte {
+func tcp4Packet(srcIPPort, dstIPPort netip.AddrPort, flags header.TCPFlags, segmentSize, seq uint32) []byte {
+	return tcp4PacketMutateIPFields(srcIPPort, dstIPPort, flags, segmentSize, seq, nil)
+}
+
+func tcp6PacketMutateIPFields(srcIPPort, dstIPPort netip.AddrPort, flags header.TCPFlags, segmentSize, seq uint32, ipFn func(*header.IPv6Fields)) []byte {
 	totalLen := 60 + segmentSize
 	b := make([]byte, offset+int(totalLen), 65535)
 	ipv6H := header.IPv6(b[offset:])
 	srcAs16 := srcIPPort.Addr().As16()
 	dstAs16 := dstIPPort.Addr().As16()
-	ipv6H.Encode(&header.IPv6Fields{
+	ipFields := &header.IPv6Fields{
 		SrcAddr:           tcpip.Address(srcAs16[:]),
 		DstAddr:           tcpip.Address(dstAs16[:]),
 		TransportProtocol: unix.IPPROTO_TCP,
 		HopLimit:          64,
 		PayloadLength:     uint16(segmentSize + 20),
-	})
+	}
+	if ipFn != nil {
+		ipFn(ipFields)
+	}
+	ipv6H.Encode(ipFields)
 	tcpH := header.TCP(b[offset+40:])
 	tcpH.Encode(&header.TCPFields{
 		SrcPort:    srcIPPort.Port(),
@@ -85,6 +97,10 @@ func tcp6Packet(srcIPPort, dstIPPort netip.AddrPort, flags header.TCPFlags, segm
 	return b
 }
 
+func tcp6Packet(srcIPPort, dstIPPort netip.AddrPort, flags header.TCPFlags, segmentSize, seq uint32) []byte {
+	return tcp6PacketMutateIPFields(srcIPPort, dstIPPort, flags, segmentSize, seq, nil)
+}
+
 func Test_handleVirtioRead(t *testing.T) {
 	tests := []struct {
 		name     string
@@ -245,6 +261,78 @@ func Test_handleGRO(t *testing.T) {
 			[]int{340},
 			false,
 		},
+		{
+			"tcp4 unequal TTL",
+			[][]byte{
+				tcp4Packet(ip4PortA, ip4PortB, header.TCPFlagAck, 100, 1),
+				tcp4PacketMutateIPFields(ip4PortA, ip4PortB, header.TCPFlagAck, 100, 101, func(fields *header.IPv4Fields) {
+					fields.TTL++
+				}),
+			},
+			[]int{0, 1},
+			[]int{140, 140},
+			false,
+		},
+		{
+			"tcp4 unequal ToS",
+			[][]byte{
+				tcp4Packet(ip4PortA, ip4PortB, header.TCPFlagAck, 100, 1),
+				tcp4PacketMutateIPFields(ip4PortA, ip4PortB, header.TCPFlagAck, 100, 101, func(fields *header.IPv4Fields) {
+					fields.TOS++
+				}),
+			},
+			[]int{0, 1},
+			[]int{140, 140},
+			false,
+		},
+		{
+			"tcp4 unequal flags more fragments set",
+			[][]byte{
+				tcp4Packet(ip4PortA, ip4PortB, header.TCPFlagAck, 100, 1),
+				tcp4PacketMutateIPFields(ip4PortA, ip4PortB, header.TCPFlagAck, 100, 101, func(fields *header.IPv4Fields) {
+					fields.Flags = 1
+				}),
+			},
+			[]int{0, 1},
+			[]int{140, 140},
+			false,
+		},
+		{
+			"tcp4 unequal flags DF set",
+			[][]byte{
+				tcp4Packet(ip4PortA, ip4PortB, header.TCPFlagAck, 100, 1),
+				tcp4PacketMutateIPFields(ip4PortA, ip4PortB, header.TCPFlagAck, 100, 101, func(fields *header.IPv4Fields) {
+					fields.Flags = 2
+				}),
+			},
+			[]int{0, 1},
+			[]int{140, 140},
+			false,
+		},
+		{
+			"tcp6 unequal hop limit",
+			[][]byte{
+				tcp6Packet(ip6PortA, ip6PortB, header.TCPFlagAck, 100, 1),
+				tcp6PacketMutateIPFields(ip6PortA, ip6PortB, header.TCPFlagAck, 100, 101, func(fields *header.IPv6Fields) {
+					fields.HopLimit++
+				}),
+			},
+			[]int{0, 1},
+			[]int{160, 160},
+			false,
+		},
+		{
+			"tcp6 unequal traffic class",
+			[][]byte{
+				tcp6Packet(ip6PortA, ip6PortB, header.TCPFlagAck, 100, 1),
+				tcp6PacketMutateIPFields(ip6PortA, ip6PortB, header.TCPFlagAck, 100, 101, func(fields *header.IPv6Fields) {
+					fields.TrafficClass++
+				}),
+			},
+			[]int{0, 1},
+			[]int{160, 160},
+			false,
+		},
 	}
 
 	for _, tt := range tests {