wgengine: loop back tuntap packets destined for local IPs.

macOS incorrectly sends packets for the local Tailscale IP
into our tunnel interface. We have to turn the packets around
and send them back to the kernel.

Fixes tailscale/corp#189.

Signed-off-by: David Anderson <danderson@tailscale.com>

wgengine/userspace.go

@@ -18,6 +18,7 @@ import (
 	"runtime"
 	"strings"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	"github.com/tailscale/wireguard-go/device"
@@ -49,15 +50,21 @@ import (
 const minimalMTU = 1280
 
 type userspaceEngine struct {
-	logf      logger.Logf
-	reqCh     chan struct{}
-	waitCh    chan struct{} // chan is closed when first Close call completes; contrast with closing bool
-	tundev    *tstun.TUN
-	wgdev     *device.Device
-	router    router.Router
-	resolver  *tsdns.Resolver
-	magicConn *magicsock.Conn
-	linkMon   *monitor.Mon
+	logf            logger.Logf
+	reqCh           chan struct{}
+	waitCh          chan struct{} // chan is closed when first Close call completes; contrast with closing bool
+	tundev          *tstun.TUN
+	wgdev           *device.Device
+	router          router.Router
+	resolver        *tsdns.Resolver
+	useTailscaleDNS bool
+	magicConn       *magicsock.Conn
+	linkMon         *monitor.Mon
+
+	// localAddrs is the set of IP addresses assigned to the local
+	// tunnel interface. It's used to reflect local packets
+	// incorrectly sent to us.
+	localAddrs atomic.Value // of map[packet.IP]bool
 
 	wgLock        sync.Mutex // serializes all wgdev operations; see lock order comment below
 	lastEngineSig string
@@ -161,22 +168,22 @@ func newUserspaceEngineAdvanced(conf EngineConfig) (_ Engine, reterr error) {
 	logf := conf.Logf
 
 	e := &userspaceEngine{
-		logf:     logf,
-		reqCh:    make(chan struct{}, 1),
-		waitCh:   make(chan struct{}),
-		tundev:   tstun.WrapTUN(logf, conf.TUN),
-		resolver: tsdns.NewResolver(logf),
-		pingers:  make(map[wgcfg.Key]*pinger),
-	}
+		logf:            logf,
+		reqCh:           make(chan struct{}, 1),
+		waitCh:          make(chan struct{}),
+		tundev:          tstun.WrapTUN(logf, conf.TUN),
+		resolver:        tsdns.NewResolver(logf),
+		useTailscaleDNS: conf.UseTailscaleDNS,
+		pingers:         make(map[wgcfg.Key]*pinger),
+	}
+	e.localAddrs.Store(map[packet.IP]bool{})
 	e.linkState, _ = getLinkState()
 
 	// Respond to all pings only in fake mode.
 	if conf.EchoRespondToAll {
 		e.tundev.PostFilterIn = echoRespondToAll
 	}
-	if conf.UseTailscaleDNS {
-		e.tundev.PreFilterOut = e.handleDNS
-	}
+	e.tundev.PreFilterOut = e.handleLocalPackets
 
 	mon, err := monitor.New(logf, func() { e.LinkChange(false) })
 	if err != nil {
@@ -314,6 +321,40 @@ func echoRespondToAll(p *packet.ParsedPacket, t *tstun.TUN) filter.Response {
 	return filter.Accept
 }
 
+// handleLocalPackets inspects packets coming from the local network
+// stack, and intercepts any packets that should be handled by
+// tailscaled directly. Other packets are allowed to proceed into the
+// main ACL filter.
+func (e *userspaceEngine) handleLocalPackets(p *packet.ParsedPacket, t *tstun.TUN) filter.Response {
+	if e.useTailscaleDNS {
+		if verdict := e.handleDNS(p, t); verdict == filter.Drop {
+			// local DNS handled the packet.
+			return filter.Drop
+		}
+	}
+
+	if runtime.GOOS == "darwin" && e.isLocalAddr(p.DstIP) {
+		// macOS NetworkExtension directs packets destined to the
+		// tunnel's local IP address into the tunnel, instead of
+		// looping back within the kernel network stack. We have to
+		// notice that an outbound packet is actually destined for
+		// ourselves, and loop it back into macOS.
+		t.InjectInboundCopy(p.Buffer())
+		return filter.Drop
+	}
+
+	return filter.Accept
+}
+
+func (e *userspaceEngine) isLocalAddr(ip packet.IP) bool {
+	localAddrs, ok := e.localAddrs.Load().(map[packet.IP]bool)
+	if !ok {
+		e.logf("[unexpected] e.localAddrs was nil, can't check for loopback packet")
+		return false
+	}
+	return localAddrs[ip]
+}
+
 // handleDNS is an outbound pre-filter resolving Tailscale domains.
 func (e *userspaceEngine) handleDNS(p *packet.ParsedPacket, t *tstun.TUN) filter.Response {
 	if e.resolver.AcceptsPacket(p) {
@@ -460,6 +501,17 @@ func (e *userspaceEngine) Reconfig(cfg *wgcfg.Config, routerCfg *router.Config)
 		panic("routerCfg must not be nil")
 	}
 
+	localAddrs := map[packet.IP]bool{}
+	for _, addr := range routerCfg.LocalAddrs {
+		// TODO: ipv6
+		if !addr.IP.Is4() {
+			continue
+		}
+		bs := addr.IP.As16()
+		localAddrs[packet.NewIP(net.IP(bs[12:16]))] = true
+	}
+	e.localAddrs.Store(localAddrs)
+
 	e.wgLock.Lock()
 	defer e.wgLock.Unlock()