wgengine/magicsock: start to make disco reception Geneve aware (#15832)

Updates tailscale/corp#27502

Signed-off-by: Jordan Whited <jordan@tailscale.com>

wgengine/magicsock/magicsock.go

@@ -9,6 +9,7 @@ import (
 	"bufio"
 	"bytes"
 	"context"
+	"encoding/binary"
 	"errors"
 	"expvar"
 	"fmt"
@@ -1707,6 +1708,45 @@ const (
 	discoRXPathRawSocket discoRXPath = "raw socket"
 )
 
+const discoHeaderLen = len(disco.Magic) + key.DiscoPublicRawLen
+
+// isDiscoMaybeGeneve reports whether msg is a Tailscale Disco protocol
+// message, and if true, whether it is encapsulated by a Geneve header.
+//
+// isGeneveEncap is only relevant when isDiscoMsg is true.
+//
+// Naked Disco, Geneve followed by Disco, and naked WireGuard can be confidently
+// distinguished based on the following:
+//  1. [disco.Magic] is sufficiently non-overlapping with a Geneve protocol
+//     field value of [packet.GeneveProtocolDisco].
+//  2. [disco.Magic] is sufficiently non-overlapping with the first 4 bytes of
+//     a WireGuard packet.
+//  3. [packet.GeneveHeader] with a Geneve protocol field value of
+//     [packet.GeneveProtocolDisco] is sufficiently non-overlapping with the
+//     first 4 bytes of a WireGuard packet.
+func isDiscoMaybeGeneve(msg []byte) (isDiscoMsg bool, isGeneveEncap bool) {
+	if len(msg) < discoHeaderLen {
+		return false, false
+	}
+	if string(msg[:len(disco.Magic)]) == disco.Magic {
+		return true, false
+	}
+	if len(msg) < packet.GeneveFixedHeaderLength+discoHeaderLen {
+		return false, false
+	}
+	if msg[0]&0xC0 != 0 || // version bits that we always transmit as 0s
+		msg[1]&0x3F != 0 || // reserved bits that we always transmit as 0s
+		binary.BigEndian.Uint16(msg[2:4]) != packet.GeneveProtocolDisco ||
+		msg[7] != 0 { // reserved byte that we always transmit as 0
+		return false, false
+	}
+	msg = msg[packet.GeneveFixedHeaderLength:]
+	if string(msg[:len(disco.Magic)]) == disco.Magic {
+		return true, true
+	}
+	return false, false
+}
+
 // handleDiscoMessage handles a discovery message and reports whether
 // msg was a Tailscale inter-node discovery message.
 //
@@ -1722,18 +1762,16 @@ const (
 // it was received from at the DERP layer. derpNodeSrc is zero when received
 // over UDP.
 func (c *Conn) handleDiscoMessage(msg []byte, src netip.AddrPort, derpNodeSrc key.NodePublic, via discoRXPath) (isDiscoMsg bool) {
-	const headerLen = len(disco.Magic) + key.DiscoPublicRawLen
-	if len(msg) < headerLen || string(msg[:len(disco.Magic)]) != disco.Magic {
-		return false
+	isDiscoMsg, isGeneveEncap := isDiscoMaybeGeneve(msg)
+	if !isDiscoMsg {
+		return
+	}
+	if isGeneveEncap {
+		// TODO(jwhited): decode Geneve header
+		msg = msg[packet.GeneveFixedHeaderLength:]
 	}
 
-	// If the first four parts are the prefix of disco.Magic
-	// (0x5453f09f) then it's definitely not a valid WireGuard
-	// packet (which starts with little-endian uint32 1, 2, 3, 4).
-	// Use naked returns for all following paths.
-	isDiscoMsg = true
-
-	sender := key.DiscoPublicFromRaw32(mem.B(msg[len(disco.Magic):headerLen]))
+	sender := key.DiscoPublicFromRaw32(mem.B(msg[len(disco.Magic):discoHeaderLen]))
 
 	c.mu.Lock()
 	defer c.mu.Unlock()
@@ -1751,6 +1789,10 @@ func (c *Conn) handleDiscoMessage(msg []byte, src netip.AddrPort, derpNodeSrc ke
 	}
 
 	if !c.peerMap.knownPeerDiscoKey(sender) {
+		// Geneve encapsulated disco used for udp relay handshakes are not known
+		// "peer" keys as they are dynamically discovered by UDP relay endpoint
+		// allocation or [disco.CallMeMaybeVia] reception.
+		// TODO(jwhited): handle relay handshake messsages instead of early return
 		metricRecvDiscoBadPeer.Add(1)
 		if debugDisco() {
 			c.logf("magicsock: disco: ignoring disco-looking frame, don't know of key %v", sender.ShortString())
@@ -1774,7 +1816,7 @@ func (c *Conn) handleDiscoMessage(msg []byte, src netip.AddrPort, derpNodeSrc ke
 
 	di := c.discoInfoLocked(sender)
 
-	sealedBox := msg[headerLen:]
+	sealedBox := msg[discoHeaderLen:]
 	payload, ok := di.sharedKey.Open(sealedBox)
 	if !ok {
 		// This might be have been intended for a previous

wgengine/magicsock/magicsock_test.go

@@ -3155,3 +3155,165 @@ func TestNetworkDownSendErrors(t *testing.T) {
 		t.Errorf("expected NetworkDown to increment packet dropped metric; got %q", resp.Body.String())
 	}
 }
+
+func Test_isDiscoMaybeGeneve(t *testing.T) {
+	discoPub := key.DiscoPublicFromRaw32(mem.B([]byte{1: 1, 30: 30, 31: 31}))
+	nakedDisco := make([]byte, 0, 512)
+	nakedDisco = append(nakedDisco, disco.Magic...)
+	nakedDisco = discoPub.AppendTo(nakedDisco)
+
+	geneveEncapDisco := make([]byte, packet.GeneveFixedHeaderLength+len(nakedDisco))
+	gh := packet.GeneveHeader{
+		Version:  0,
+		Protocol: packet.GeneveProtocolDisco,
+		VNI:      1,
+		Control:  true,
+	}
+	err := gh.Encode(geneveEncapDisco)
+	if err != nil {
+		t.Fatal(err)
+	}
+	copy(geneveEncapDisco[packet.GeneveFixedHeaderLength:], nakedDisco)
+
+	nakedWireGuardInitiation := make([]byte, len(geneveEncapDisco))
+	binary.LittleEndian.PutUint32(nakedWireGuardInitiation, device.MessageInitiationType)
+	nakedWireGuardResponse := make([]byte, len(geneveEncapDisco))
+	binary.LittleEndian.PutUint32(nakedWireGuardResponse, device.MessageResponseType)
+	nakedWireGuardCookieReply := make([]byte, len(geneveEncapDisco))
+	binary.LittleEndian.PutUint32(nakedWireGuardCookieReply, device.MessageCookieReplyType)
+	nakedWireGuardTransport := make([]byte, len(geneveEncapDisco))
+	binary.LittleEndian.PutUint32(nakedWireGuardTransport, device.MessageTransportType)
+
+	geneveEncapWireGuard := make([]byte, packet.GeneveFixedHeaderLength+len(nakedWireGuardInitiation))
+	gh = packet.GeneveHeader{
+		Version:  0,
+		Protocol: packet.GeneveProtocolWireGuard,
+		VNI:      1,
+		Control:  true,
+	}
+	err = gh.Encode(geneveEncapWireGuard)
+	if err != nil {
+		t.Fatal(err)
+	}
+	copy(geneveEncapWireGuard[packet.GeneveFixedHeaderLength:], nakedWireGuardInitiation)
+
+	geneveEncapDiscoNonZeroGeneveVersion := make([]byte, packet.GeneveFixedHeaderLength+len(nakedDisco))
+	gh = packet.GeneveHeader{
+		Version:  1,
+		Protocol: packet.GeneveProtocolDisco,
+		VNI:      1,
+		Control:  true,
+	}
+	err = gh.Encode(geneveEncapDiscoNonZeroGeneveVersion)
+	if err != nil {
+		t.Fatal(err)
+	}
+	copy(geneveEncapDiscoNonZeroGeneveVersion[packet.GeneveFixedHeaderLength:], nakedDisco)
+
+	geneveEncapDiscoNonZeroGeneveReservedBits := make([]byte, packet.GeneveFixedHeaderLength+len(nakedDisco))
+	gh = packet.GeneveHeader{
+		Version:  0,
+		Protocol: packet.GeneveProtocolDisco,
+		VNI:      1,
+		Control:  true,
+	}
+	err = gh.Encode(geneveEncapDiscoNonZeroGeneveReservedBits)
+	if err != nil {
+		t.Fatal(err)
+	}
+	geneveEncapDiscoNonZeroGeneveReservedBits[1] |= 0x3F
+	copy(geneveEncapDiscoNonZeroGeneveReservedBits[packet.GeneveFixedHeaderLength:], nakedDisco)
+
+	geneveEncapDiscoNonZeroGeneveVNILSB := make([]byte, packet.GeneveFixedHeaderLength+len(nakedDisco))
+	gh = packet.GeneveHeader{
+		Version:  0,
+		Protocol: packet.GeneveProtocolDisco,
+		VNI:      1,
+		Control:  true,
+	}
+	err = gh.Encode(geneveEncapDiscoNonZeroGeneveVNILSB)
+	if err != nil {
+		t.Fatal(err)
+	}
+	geneveEncapDiscoNonZeroGeneveVNILSB[7] |= 0xFF
+	copy(geneveEncapDiscoNonZeroGeneveVNILSB[packet.GeneveFixedHeaderLength:], nakedDisco)
+
+	tests := []struct {
+		name              string
+		msg               []byte
+		wantIsDiscoMsg    bool
+		wantIsGeneveEncap bool
+	}{
+		{
+			name:              "naked disco",
+			msg:               nakedDisco,
+			wantIsDiscoMsg:    true,
+			wantIsGeneveEncap: false,
+		},
+		{
+			name:              "geneve encap disco",
+			msg:               geneveEncapDisco,
+			wantIsDiscoMsg:    true,
+			wantIsGeneveEncap: true,
+		},
+		{
+			name:              "geneve encap disco nonzero geneve version",
+			msg:               geneveEncapDiscoNonZeroGeneveVersion,
+			wantIsDiscoMsg:    false,
+			wantIsGeneveEncap: false,
+		},
+		{
+			name:              "geneve encap disco nonzero geneve reserved bits",
+			msg:               geneveEncapDiscoNonZeroGeneveReservedBits,
+			wantIsDiscoMsg:    false,
+			wantIsGeneveEncap: false,
+		},
+		{
+			name:              "geneve encap disco nonzero geneve vni lsb",
+			msg:               geneveEncapDiscoNonZeroGeneveVNILSB,
+			wantIsDiscoMsg:    false,
+			wantIsGeneveEncap: false,
+		},
+		{
+			name:              "geneve encap wireguard",
+			msg:               geneveEncapWireGuard,
+			wantIsDiscoMsg:    false,
+			wantIsGeneveEncap: false,
+		},
+		{
+			name:              "naked WireGuard Initiation type",
+			msg:               nakedWireGuardInitiation,
+			wantIsDiscoMsg:    false,
+			wantIsGeneveEncap: false,
+		},
+		{
+			name:              "naked WireGuard Response type",
+			msg:               nakedWireGuardResponse,
+			wantIsDiscoMsg:    false,
+			wantIsGeneveEncap: false,
+		},
+		{
+			name:              "naked WireGuard Cookie Reply type",
+			msg:               nakedWireGuardCookieReply,
+			wantIsDiscoMsg:    false,
+			wantIsGeneveEncap: false,
+		},
+		{
+			name:              "naked WireGuard Transport type",
+			msg:               nakedWireGuardTransport,
+			wantIsDiscoMsg:    false,
+			wantIsGeneveEncap: false,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			gotIsDiscoMsg, gotIsGeneveEncap := isDiscoMaybeGeneve(tt.msg)
+			if gotIsDiscoMsg != tt.wantIsDiscoMsg {
+				t.Errorf("isDiscoMaybeGeneve() gotIsDiscoMsg = %v, want %v", gotIsDiscoMsg, tt.wantIsDiscoMsg)
+			}
+			if gotIsGeneveEncap != tt.wantIsGeneveEncap {
+				t.Errorf("isDiscoMaybeGeneve() gotIsGeneveEncap = %v, want %v", gotIsGeneveEncap, tt.wantIsGeneveEncap)
+			}
+		})
+	}
+}