device: use atomic access for unlocked keypair.next

Go's GC semantics might not always guarantee the safety of this, and the
race detector gets upset too, so instead we wrap this all in atomic
accessors.

Reported-by: David Anderson <danderson@tailscale.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

device/keypair.go

@@ -8,7 +8,9 @@ package device
 import (
 	"crypto/cipher"
 	"sync"
+	"sync/atomic"
 	"time"
+	"unsafe"
 
 	"golang.zx2c4.com/wireguard/replay"
 )
@@ -38,6 +40,14 @@ type Keypairs struct {
 	next     *Keypair
 }
 
+func (kp *Keypairs) storeNext(next *Keypair) {
+	atomic.StorePointer((*unsafe.Pointer)((unsafe.Pointer)(&kp.next)), (unsafe.Pointer)(next))
+}
+
+func (kp *Keypairs) loadNext() *Keypair {
+	return (*Keypair)(atomic.LoadPointer((*unsafe.Pointer)((unsafe.Pointer)(&kp.next))))
+}
+
 func (kp *Keypairs) Current() *Keypair {
 	kp.RLock()
 	defer kp.RUnlock()

device/noise-protocol.go

@@ -14,6 +14,7 @@ import (
 	"golang.org/x/crypto/blake2s"
 	"golang.org/x/crypto/chacha20poly1305"
 	"golang.org/x/crypto/poly1305"
+
 	"golang.zx2c4.com/wireguard/tai64n"
 )
 
@@ -583,12 +584,12 @@ func (peer *Peer) BeginSymmetricSession() error {
 	defer keypairs.Unlock()
 
 	previous := keypairs.previous
-	next := keypairs.next
+	next := keypairs.loadNext()
 	current := keypairs.current
 
 	if isInitiator {
 		if next != nil {
-			keypairs.next = nil
+			keypairs.storeNext(nil)
 			keypairs.previous = next
 			device.DeleteKeypair(current)
 		} else {
@@ -597,7 +598,7 @@ func (peer *Peer) BeginSymmetricSession() error {
 		device.DeleteKeypair(previous)
 		keypairs.current = keypair
 	} else {
-		keypairs.next = keypair
+		keypairs.storeNext(keypair)
 		device.DeleteKeypair(next)
 		keypairs.previous = nil
 		device.DeleteKeypair(previous)
@@ -608,18 +609,19 @@ func (peer *Peer) BeginSymmetricSession() error {
 
 func (peer *Peer) ReceivedWithKeypair(receivedKeypair *Keypair) bool {
 	keypairs := &peer.keypairs
-	if keypairs.next != receivedKeypair {
+
+	if keypairs.loadNext() != receivedKeypair {
 		return false
 	}
 	keypairs.Lock()
 	defer keypairs.Unlock()
-	if keypairs.next != receivedKeypair {
+	if keypairs.loadNext() != receivedKeypair {
 		return false
 	}
 	old := keypairs.previous
 	keypairs.previous = keypairs.current
 	peer.device.DeleteKeypair(old)
-	keypairs.current = keypairs.next
+	keypairs.current = keypairs.loadNext()
-	keypairs.next = nil
+	keypairs.storeNext(nil)
 	return true
 }

device/noise_test.go

@@ -113,7 +113,7 @@ func TestNoiseHandshake(t *testing.T) {
 		t.Fatal("failed to derive keypair for peer 2", err)
 	}
 
-	key1 := peer1.keypairs.next
+	key1 := peer1.keypairs.loadNext()
 	key2 := peer2.keypairs.current
 
 	// encrypting / decryption test

device/peer.go

@@ -223,10 +223,10 @@ func (peer *Peer) ZeroAndFlushAll() {
 	keypairs.Lock()
 	device.DeleteKeypair(keypairs.previous)
 	device.DeleteKeypair(keypairs.current)
-	device.DeleteKeypair(keypairs.next)
+	device.DeleteKeypair(keypairs.loadNext())
 	keypairs.previous = nil
 	keypairs.current = nil
-	keypairs.next = nil
+	keypairs.storeNext(nil)
 	keypairs.Unlock()
 
 	// clear handshake state
@@ -254,7 +254,7 @@ func (peer *Peer) ExpireCurrentKeypairs() {
 		keypairs.current.sendNonce = RejectAfterMessages
 	}
 	if keypairs.next != nil {
-		keypairs.next.sendNonce = RejectAfterMessages
+		keypairs.loadNext().sendNonce = RejectAfterMessages
 	}
 	keypairs.Unlock()
 }