device: fix races from changing private_key

Access keypair.sendNonce atomically.
Eliminate one unnecessary initialization to zero.

Mutate handshake.lastSentHandshake with the mutex held.

Co-authored-by: David Anderson <danderson@tailscale.com>
Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

device/device_test.go

@@ -215,6 +215,14 @@ func TestConcurrencySafety(t *testing.T) {
 	}()
 	warmup.Wait()
 
+	applyCfg := func(cfg io.ReadSeeker) {
+		cfg.Seek(0, io.SeekStart)
+		err := pair[0].dev.IpcSetOperation(cfg)
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+
 	// Change persistent_keepalive_interval concurrently with tunnel use.
 	t.Run("persistentKeepaliveInterval", func(t *testing.T) {
 		cfg := uapiCfg(
@@ -222,11 +230,24 @@ func TestConcurrencySafety(t *testing.T) {
 			"persistent_keepalive_interval", "1",
 		)
 		for i := 0; i < 1000; i++ {
-			cfg.Seek(0, io.SeekStart)
+			applyCfg(cfg)
-			err := pair[0].dev.IpcSetOperation(cfg)
-			if err != nil {
-				t.Fatal(err)
 		}
+	})
+
+	// Change private keys concurrently with tunnel use.
+	t.Run("privateKey", func(t *testing.T) {
+		bad := uapiCfg("private_key", "7777777777777777777777777777777777777777777777777777777777777777")
+		good := uapiCfg("private_key", "481eb0d8113a4a5da532d2c3e9c14b53c8454b34ab109676f6b58c2245e37b58")
+		// Set iters to a large number like 1000 to flush out data races quickly.
+		// Don't leave it large. That can cause logical races
+		// in which the handshake is interleaved with key changes
+		// such that the private key appears to be unchanging but
+		// other state gets reset, which can cause handshake failures like
+		// "Received packet with invalid mac1".
+		const iters = 1
+		for i := 0; i < iters; i++ {
+			applyCfg(bad)
+			applyCfg(good)
 		}
 	})
 

device/keypair.go

@@ -23,7 +23,7 @@ import (
  */
 
 type Keypair struct {
-	sendNonce    uint64
+	sendNonce    uint64 // accessed atomically
 	send         cipher.AEAD
 	receive      cipher.AEAD
 	replayFilter replay.Filter

device/noise-protocol.go

@@ -566,7 +566,6 @@ func (peer *Peer) BeginSymmetricSession() error {
 	setZero(recvKey[:])
 
 	keypair.created = time.Now()
-	keypair.sendNonce = 0
 	keypair.replayFilter.Reset()
 	keypair.isInitiator = isInitiator
 	keypair.localIndex = peer.handshake.localIndex

device/peer.go

@@ -249,16 +249,17 @@ func (peer *Peer) ExpireCurrentKeypairs() {
 	handshake.mutex.Lock()
 	peer.device.indexTable.Delete(handshake.localIndex)
 	handshake.Clear()
-	handshake.mutex.Unlock()
 	peer.handshake.lastSentHandshake = time.Now().Add(-(RekeyTimeout + time.Second))
+	handshake.mutex.Unlock()
 
 	keypairs := &peer.keypairs
 	keypairs.Lock()
 	if keypairs.current != nil {
-		keypairs.current.sendNonce = RejectAfterMessages
+		atomic.StoreUint64(&keypairs.current.sendNonce, RejectAfterMessages)
 	}
 	if keypairs.next != nil {
-		keypairs.loadNext().sendNonce = RejectAfterMessages
+		next := keypairs.loadNext()
+		atomic.StoreUint64(&next.sendNonce, RejectAfterMessages)
 	}
 	keypairs.Unlock()
 }

device/send.go

@@ -403,7 +403,7 @@ NextPacket:
 				// check validity of newest key pair
 
 				keypair = peer.keypairs.Current()
-				if keypair != nil && keypair.sendNonce < RejectAfterMessages {
+				if keypair != nil && atomic.LoadUint64(&keypair.sendNonce) < RejectAfterMessages {
 					if time.Since(keypair.created) < RejectAfterTime {
 						break
 					}