net/tstun: fix data races

To remove some multi-case selects, we intentionally allowed
sends on closed channels (cc23049cd2).

However, we also introduced concurrent sends and closes,
which is a data race.

This commit fixes the data race. The mutexes here are uncontended,
and thus very cheap.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

net/tstun/wrap.go

@@ -71,6 +71,9 @@ type Wrapper struct {
 	// buffer stores the oldest unconsumed packet from tdev.
 	// It is made a static buffer in order to avoid allocations.
 	buffer [maxBufferSize]byte
+	// bufferConsumedMu protects bufferConsumed from concurrent sends and closes.
+	// It does not prevent send-after-close, only data races.
+	bufferConsumedMu sync.Mutex
 	// bufferConsumed synchronizes access to buffer (shared by Read and poll).
 	//
 	// Close closes bufferConsumed. There may be outstanding sends to bufferConsumed
@@ -80,6 +83,9 @@ type Wrapper struct {
 
 	// closed signals poll (by closing) when the device is closed.
 	closed chan struct{}
+	// outboundMu protects outbound from concurrent sends and closes.
+	// It does not prevent send-after-close, only data races.
+	outboundMu sync.Mutex
 	// outbound is the queue by which packets leave the TUN device.
 	//
 	// The directions are relative to the network, not the device:
@@ -174,8 +180,12 @@ func (t *Wrapper) Close() error {
 	var err error
 	t.closeOnce.Do(func() {
 		close(t.closed)
+		t.bufferConsumedMu.Lock()
 		close(t.bufferConsumed)
+		t.bufferConsumedMu.Unlock()
+		t.outboundMu.Lock()
 		close(t.outbound)
+		t.outboundMu.Unlock()
 		err = t.tdev.Close()
 	})
 	return err
@@ -275,7 +285,6 @@ func allowSendOnClosedChannel() {
 // This is needed because t.tdev.Read in general may block (it does on Windows),
 // so packets may be stuck in t.outbound if t.Read called t.tdev.Read directly.
 func (t *Wrapper) poll() {
-	defer allowSendOnClosedChannel() // for send to t.outbound
 	for range t.bufferConsumed {
 		var n int
 		var err error
@@ -293,10 +302,28 @@ func (t *Wrapper) poll() {
 			}
 			n, err = t.tdev.Read(t.buffer[:], PacketStartOffset)
 		}
-		t.outbound <- tunReadResult{data: t.buffer[PacketStartOffset : PacketStartOffset+n], err: err}
+		t.sendOutbound(tunReadResult{data: t.buffer[PacketStartOffset : PacketStartOffset+n], err: err})
 	}
 }
 
+// sendBufferConsumed does t.bufferConsumed <- struct{}{}.
+// It protects against any panics or data races that that send could cause.
+func (t *Wrapper) sendBufferConsumed() {
+	defer allowSendOnClosedChannel()
+	t.bufferConsumedMu.Lock()
+	defer t.bufferConsumedMu.Unlock()
+	t.bufferConsumed <- struct{}{}
+}
+
+// sendOutbound does t.outboundMu <- r.
+// It protects against any panics or data races that that send could cause.
+func (t *Wrapper) sendOutbound(r tunReadResult) {
+	defer allowSendOnClosedChannel()
+	t.outboundMu.Lock()
+	defer t.outboundMu.Unlock()
+	t.outbound <- r
+}
+
 var magicDNSIPPort = netaddr.MustParseIPPort("100.100.100.100:0")
 
 func (t *Wrapper) filterOut(p *packet.Parsed) filter.Response {
@@ -357,7 +384,6 @@ func (t *Wrapper) Read(buf []byte, offset int) (int, error) {
 	if res.err != nil {
 		return 0, res.err
 	}
-	defer allowSendOnClosedChannel() // for send to t.bufferConsumed
 	pkt := res.data
 	n := copy(buf[offset:], pkt)
 	// t.buffer has a fixed location in memory.
@@ -366,7 +392,7 @@ func (t *Wrapper) Read(buf []byte, offset int) (int, error) {
 	isInjectedPacket := &pkt[0] != &t.buffer[PacketStartOffset]
 	if !isInjectedPacket {
 		// We are done with t.buffer. Let poll re-use it.
-		t.bufferConsumed <- struct{}{}
+		t.sendBufferConsumed()
 	}
 
 	p := parsedPacketPool.Get().(*packet.Parsed)
@@ -583,8 +609,7 @@ func (t *Wrapper) InjectOutbound(packet []byte) error {
 	if len(packet) == 0 {
 		return nil
 	}
-	defer allowSendOnClosedChannel() // for send to t.outbound
-	t.outbound <- tunReadResult{data: packet}
+	t.sendOutbound(tunReadResult{data: packet})
 	return nil
 }