From 75a7779b42164e9de250ab83158409e72054db13 Mon Sep 17 00:00:00 2001
From: Brad Fitzpatrick <bradfitz@tailscale.com>
Date: Sat, 16 Oct 2021 14:55:26 -0700
Subject: [PATCH] disco, wgengine/magicsock: send self node key in disco pings

This lets clients quickly (sub-millisecond within a local LAN) map
from an ambiguous disco key to a node key without waiting for a
CallMeMaybe (over relatively high latency DERP).

Updates #3088

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
---
 disco/disco.go                  | 26 +++++++++++++++++++++++---
 disco/disco_test.go             | 14 ++++++++++++++
 wgengine/magicsock/magicsock.go | 15 +++++++++++++--
 3 files changed, 50 insertions(+), 5 deletions(-)

diff --git a/disco/disco.go b/disco/disco.go
index d91db6a0d..b0476bba1 100644
--- a/disco/disco.go
+++ b/disco/disco.go
@@ -26,6 +26,7 @@ import (
 	"net"
 
 	"inet.af/netaddr"
+	"tailscale.com/tailcfg"
 )
 
 // Magic is the 6 byte header of all discovery messages.
@@ -106,12 +107,28 @@ func appendMsgHeader(b []byte, t MessageType, ver uint8, dataLen int) (all, data
 }
 
 type Ping struct {
+	// TxID is a random client-generated per-ping transaction ID.
 	TxID [12]byte
+
+	// NodeKey is the ping sender's wireguard public key.  Old
+	// clients (~1.16.0 and earlier) don't send this field.  It
+	// shouldn't be trusted by itself. But if present and the
+	// netmap's peer for this NodeKey's DiscoKey matches the
+	// sender of this disco key, they it can be.
+	NodeKey tailcfg.NodeKey
 }
 
 func (m *Ping) AppendMarshal(b []byte) []byte {
-	ret, d := appendMsgHeader(b, TypePing, v0, 12)
-	copy(d, m.TxID[:])
+	dataLen := 12
+	hasKey := !m.NodeKey.IsZero()
+	if hasKey {
+		dataLen += len(m.NodeKey)
+	}
+	ret, d := appendMsgHeader(b, TypePing, v0, dataLen)
+	n := copy(d, m.TxID[:])
+	if hasKey {
+		copy(d[n:], m.NodeKey[:])
+	}
 	return ret
 }
 
@@ -120,7 +137,10 @@ func parsePing(ver uint8, p []byte) (m *Ping, err error) {
 		return nil, errShort
 	}
 	m = new(Ping)
-	copy(m.TxID[:], p)
+	p = p[copy(m.TxID[:], p):]
+	if len(p) >= len(m.NodeKey) {
+		copy(m.NodeKey[:], p)
+	}
 	return m, nil
 }
 
diff --git a/disco/disco_test.go b/disco/disco_test.go
index 9b16e62ba..a02622d79 100644
--- a/disco/disco_test.go
+++ b/disco/disco_test.go
@@ -11,6 +11,7 @@ import (
 	"testing"
 
 	"inet.af/netaddr"
+	"tailscale.com/tailcfg"
 )
 
 func TestMarshalAndParse(t *testing.T) {
@@ -26,6 +27,19 @@ func TestMarshalAndParse(t *testing.T) {
 			},
 			want: "01 00 01 02 03 04 05 06 07 08 09 0a 0b 0c",
 		},
+		{
+			name: "ping_with_nodekey_src",
+			m: &Ping{
+				TxID: [12]byte{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12},
+				NodeKey: tailcfg.NodeKey{
+					1:  1,
+					2:  2,
+					30: 30,
+					31: 31,
+				},
+			},
+			want: "01 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 00 01 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 1e 1f",
+		},
 		{
 			name: "pong",
 			m: &Pong{
diff --git a/wgengine/magicsock/magicsock.go b/wgengine/magicsock/magicsock.go
index 9d16c6e8b..db9cfe00a 100644
--- a/wgengine/magicsock/magicsock.go
+++ b/wgengine/magicsock/magicsock.go
@@ -281,7 +281,8 @@ type Conn struct {
 	networkUp syncs.AtomicBool
 
 	// havePrivateKey is whether privateKey is non-zero.
-	havePrivateKey syncs.AtomicBool
+	havePrivateKey  syncs.AtomicBool
+	publicKeyAtomic atomic.Value // of tailcfg.NodeKey (or NodeKey zero value if !havePrivateKey)
 
 	// port is the preferred port from opts.Port; 0 means auto.
 	port syncs.AtomicUint32
@@ -2053,6 +2054,12 @@ func (c *Conn) SetPrivateKey(privateKey wgkey.Private) error {
 	c.privateKey = newKey
 	c.havePrivateKey.Set(!newKey.IsZero())
 
+	if newKey.IsZero() {
+		c.publicKeyAtomic.Store(tailcfg.NodeKey{})
+	} else {
+		c.publicKeyAtomic.Store(tailcfg.NodeKey(newKey.Public()))
+	}
+
 	if oldKey.IsZero() {
 		c.everHadKey = true
 		c.logf("magicsock: SetPrivateKey called (init)")
@@ -3401,7 +3408,11 @@ func (de *endpoint) removeSentPingLocked(txid stun.TxID, sp sentPing) {
 // The caller (startPingLocked) should've already been recorded the ping in
 // sentPing and set up the timer.
 func (de *endpoint) sendDiscoPing(ep netaddr.IPPort, txid stun.TxID, logLevel discoLogLevel) {
-	sent, _ := de.sendDiscoMessage(ep, &disco.Ping{TxID: [12]byte(txid)}, logLevel)
+	selfPubKey, _ := de.c.publicKeyAtomic.Load().(tailcfg.NodeKey)
+	sent, _ := de.sendDiscoMessage(ep, &disco.Ping{
+		TxID:    [12]byte(txid),
+		NodeKey: selfPubKey,
+	}, logLevel)
 	if !sent {
 		de.forgetPing(txid)
 	}