cmd/tailscaled, wgengine/netstack: add start of gvisor userspace netstack work

Not usefully functional yet (mostly a proof of concept), but getting
it submitted for some work @namansood is going to do atop this.

Updates #707
Updates #634
Updates #48
Updates #835

wgengine/netstack/netstack.go

@@ -0,0 +1,154 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package netstack wires up gVisor's netstack into Tailscale.
+package netstack
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"log"
+	"net"
+	"strings"
+
+	"gvisor.dev/gvisor/pkg/tcpip"
+	"gvisor.dev/gvisor/pkg/tcpip/adapters/gonet"
+	"gvisor.dev/gvisor/pkg/tcpip/buffer"
+	"gvisor.dev/gvisor/pkg/tcpip/header"
+	"gvisor.dev/gvisor/pkg/tcpip/link/channel"
+	"gvisor.dev/gvisor/pkg/tcpip/network/ipv4"
+	"gvisor.dev/gvisor/pkg/tcpip/stack"
+	"gvisor.dev/gvisor/pkg/tcpip/transport/icmp"
+	"gvisor.dev/gvisor/pkg/tcpip/transport/tcp"
+	"gvisor.dev/gvisor/pkg/tcpip/transport/udp"
+	"gvisor.dev/gvisor/pkg/waiter"
+	"inet.af/netaddr"
+	"tailscale.com/net/packet"
+	"tailscale.com/types/logger"
+	"tailscale.com/wgengine"
+	"tailscale.com/wgengine/filter"
+	"tailscale.com/wgengine/magicsock"
+	"tailscale.com/wgengine/tstun"
+)
+
+func Impl(logf logger.Logf, tundev *tstun.TUN, e wgengine.Engine, mc *magicsock.Conn) error {
+	if mc == nil {
+		return errors.New("nil magicsock.Conn")
+	}
+	if tundev == nil {
+		return errors.New("nil tundev")
+	}
+	if logf == nil {
+		return errors.New("nil logger")
+	}
+	if e == nil {
+		return errors.New("nil Engine")
+	}
+	ipstack := stack.New(stack.Options{
+		NetworkProtocols:   []stack.NetworkProtocol{ipv4.NewProtocol()},
+		TransportProtocols: []stack.TransportProtocol{tcp.NewProtocol(), udp.NewProtocol(), icmp.NewProtocol4()},
+	})
+
+	const mtu = 1500
+	linkEP := channel.New(512, mtu, "")
+
+	const nicID = 1
+	if err := ipstack.CreateNIC(nicID, linkEP); err != nil {
+		log.Fatal(err)
+	}
+
+	ipstack.AddAddress(nicID, ipv4.ProtocolNumber, tcpip.Address(net.ParseIP("100.96.188.101").To4()))
+
+	// Add 0.0.0.0/0 default route.
+	subnet, _ := tcpip.NewSubnet(tcpip.Address(strings.Repeat("\x00", 4)), tcpip.AddressMask(strings.Repeat("\x00", 4)))
+	ipstack.SetRouteTable([]tcpip.Route{
+		{
+			Destination: subnet,
+			NIC:         nicID,
+		},
+	})
+
+	// use Forwarder to accept any connection from stack
+	fwd := tcp.NewForwarder(ipstack, 0, 16, func(r *tcp.ForwarderRequest) {
+		logf("XXX ForwarderRequest: %v", r)
+		var wq waiter.Queue
+		ep, err := r.CreateEndpoint(&wq)
+		if err != nil {
+			r.Complete(true)
+			return
+		}
+		r.Complete(false)
+		c := gonet.NewTCPConn(&wq, ep)
+		// TCP echo
+		go echo(c, e, mc)
+
+	})
+	ipstack.SetTransportProtocolHandler(tcp.ProtocolNumber, fwd.HandlePacket)
+
+	go func() {
+		for {
+			packetInfo, ok := linkEP.ReadContext(context.Background())
+			if !ok {
+				logf("XXX ReadContext-for-write = ok=false")
+				continue
+			}
+			pkt := packetInfo.Pkt
+			hdrNetwork := pkt.NetworkHeader()
+			hdrTransport := pkt.TransportHeader()
+
+			full := make([]byte, 0, pkt.Size())
+			full = append(full, hdrNetwork.View()...)
+			full = append(full, hdrTransport.View()...)
+			full = append(full, pkt.Data.ToView()...)
+
+			logf("XXX packet Write out: % x", full)
+			if err := tundev.InjectOutbound(full); err != nil {
+				log.Printf("netstack inject outbound: %v", err)
+				return
+			}
+
+		}
+	}()
+
+	tundev.PostFilterIn = func(p *packet.Parsed, t *tstun.TUN) filter.Response {
+		var pn tcpip.NetworkProtocolNumber
+		switch p.IPVersion {
+		case 4:
+			pn = header.IPv4ProtocolNumber
+		case 6:
+			pn = header.IPv6ProtocolNumber
+		}
+		logf("XXX packet in (from %v): % x", p.Src, p.Buffer())
+		vv := buffer.View(append([]byte(nil), p.Buffer()...)).ToVectorisedView()
+		packetBuf := stack.NewPacketBuffer(stack.PacketBufferOptions{
+			Data: vv,
+		})
+		linkEP.InjectInbound(pn, packetBuf)
+		return filter.Accept
+	}
+	return nil
+}
+
+func echo(c *gonet.TCPConn, e wgengine.Engine, mc *magicsock.Conn) {
+	defer c.Close()
+	src, _ := netaddr.FromStdIP(c.RemoteAddr().(*net.TCPAddr).IP)
+	who := ""
+	if n, u, ok := mc.WhoIs(src); ok {
+		who = fmt.Sprintf("%v from %v", u.DisplayName, n.Name)
+	}
+	fmt.Fprintf(c, "Hello, %s! Thanks for connecting to me on port %v (Try other ports too!)\nEchoing...\n",
+		who,
+		c.LocalAddr().(*net.TCPAddr).Port)
+	buf := make([]byte, 1500)
+	for {
+		n, err := c.Read(buf)
+		if err != nil {
+			log.Printf("Err: %v", err)
+			break
+		}
+		c.Write(buf[:n])
+	}
+	log.Print("Connection closed")
+}