wgengine/netstack: fix shouldProcessInbound peerapi non-SYN handling

It was eating TCP packets to peerapi ports to subnet routers.  Some of
the TCP flow's packets went onward, some got eaten.  So some TCP flows
to subnet routers, if they used an unfortunate TCP port number, got
broken.

Change-Id: Ifea036119ccfb081f4dfa18b892373416a5239f8
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/netstack/netstack.go

@@ -535,24 +535,31 @@ var viaRange = tsaddr.TailscaleViaRange()
 // WireGuard peer) should be handled by netstack.
 func (ns *Impl) shouldProcessInbound(p *packet.Parsed, t *tstun.Wrapper) bool {
 	// Handle incoming peerapi connections in netstack.
-	if ns.lb != nil && p.IPProto == ipproto.TCP {
-		var peerAPIPort uint16
 	dstIP := p.Dst.Addr()
-		if p.TCPFlags&packet.TCPSynAck == packet.TCPSyn && ns.isLocalIP(dstIP) {
-			if port, ok := ns.lb.GetPeerAPIPort(p.Dst.Addr()); ok {
+	isLocal := ns.isLocalIP(dstIP)
+
+	// Handle TCP connection to the Tailscale IP(s) in some cases:
+	if ns.lb != nil && p.IPProto == ipproto.TCP && isLocal {
+		var peerAPIPort uint16
+
+		if p.TCPFlags&packet.TCPSynAck == packet.TCPSyn {
+			if port, ok := ns.lb.GetPeerAPIPort(dstIP); ok {
 				peerAPIPort = port
 				atomic.StoreUint32(ns.peerAPIPortAtomic(dstIP), uint32(port))
 			}
 		} else {
 			peerAPIPort = uint16(atomic.LoadUint32(ns.peerAPIPortAtomic(dstIP)))
 		}
-		if p.IPProto == ipproto.TCP && p.Dst.Port() == peerAPIPort {
+		dport := p.Dst.Port()
+		if dport == peerAPIPort {
 			return true
 		}
-	}
-	if ns.isInboundTSSH(p) && ns.processSSH() {
+
+		// Also handle SSH connections, if enabled.
+		if dport == 22 && ns.lb.ShouldRunSSH() {
 			return true
 		}
+	}
 	if p.IPVersion == 6 && viaRange.Contains(p.Dst.Addr()) {
 		return ns.lb != nil && ns.lb.ShouldHandleViaIP(p.Dst.Addr())
 	}
@@ -561,7 +568,6 @@ func (ns *Impl) shouldProcessInbound(p *packet.Parsed, t *tstun.Wrapper) bool {
 		// netstack isn't used at all; don't even do an isLocalIP lookup.
 		return false
 	}
-	isLocal := ns.isLocalIP(p.Dst.Addr())
 	if ns.ProcessLocalIPs && isLocal {
 		return true
 	}
@@ -647,12 +653,6 @@ func (ns *Impl) userPing(dstIP netip.Addr, pingResPkt []byte) {
 	}
 }
 
-func (ns *Impl) isInboundTSSH(p *packet.Parsed) bool {
-	return p.IPProto == ipproto.TCP &&
-		p.Dst.Port() == 22 &&
-		ns.isLocalIP(p.Dst.Addr())
-}
-
 // injectInbound is installed as a packet hook on the 'inbound' (from a
 // WireGuard peer) path. Returning filter.Accept releases the packet to
 // continue normally (typically being delivered to the host networking stack),