net/tstun: accept peerapi connections through the filter

Fixes tailscale/corp#1545

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

ipn/ipnlocal/local.go

@@ -1366,7 +1366,7 @@ func (b *LocalBackend) getPeerAPIPortForTSMPPing(ip netaddr.IP) (port uint16, ok
 	b.mu.Lock()
 	defer b.mu.Unlock()
 	for _, pln := range b.peerAPIListeners {
-		if pln.ip.BitLen() == ip.BitLen() {
+		if pln.ip == ip {
 			return uint16(pln.port), true
 		}
 	}

net/tstun/wrap.go

@@ -115,6 +115,9 @@ type Wrapper struct {
 
 	// disableFilter disables all filtering when set. This should only be used in tests.
 	disableFilter bool
+
+	// disableTSMPRejected disables TSMP rejected responses. For tests.
+	disableTSMPRejected bool
 }
 
 func Wrap(logf logger.Logf, tdev tun.Device) *Wrapper {
@@ -351,13 +354,26 @@ func (t *Wrapper) filterIn(buf []byte) filter.Response {
 		return filter.Drop
 	}
 
-	if filt.RunIn(p, t.filterFlags) != filter.Accept {
+	outcome := filt.RunIn(p, t.filterFlags)
+
+	// Let peerapi through the filter; its ACLs are handled at L7,
+	// not at the packet level.
+	if outcome != filter.Accept &&
+		p.IPProto == ipproto.TCP &&
+		p.TCPFlags&packet.TCPSyn != 0 &&
+		t.PeerAPIPort != nil {
+		if port, ok := t.PeerAPIPort(p.Dst.IP); ok && port == p.Dst.Port {
+			outcome = filter.Accept
+		}
+	}
+
+	if outcome != filter.Accept {
 
 		// Tell them, via TSMP, we're dropping them due to the ACL.
 		// Their host networking stack can translate this into ICMP
 		// or whatnot as required. But notably, their GUI or tailscale CLI
 		// can show them a rejection history with reasons.
-		if p.IPVersion == 4 && p.IPProto == ipproto.TCP && p.TCPFlags&packet.TCPSyn != 0 {
+		if p.IPVersion == 4 && p.IPProto == ipproto.TCP && p.TCPFlags&packet.TCPSyn != 0 && !t.disableTSMPRejected {
 			rj := packet.TailscaleRejectedHeader{
 				IPSrc:  p.Dst.IP,
 				IPDst:  p.Src.IP,

net/tstun/wrap_test.go

@@ -6,6 +6,7 @@ package tstun
 
 import (
 	"bytes"
+	"encoding/binary"
 	"fmt"
 	"strconv"
 	"strings"
@@ -42,6 +43,34 @@ func udp4(src, dst string, sport, dport uint16) []byte {
 	return packet.Generate(header, []byte("udp_payload"))
 }
 
+func tcp4syn(src, dst string, sport, dport uint16) []byte {
+	sip, err := netaddr.ParseIP(src)
+	if err != nil {
+		panic(err)
+	}
+	dip, err := netaddr.ParseIP(dst)
+	if err != nil {
+		panic(err)
+	}
+	ipHeader := packet.IP4Header{
+		IPProto: ipproto.TCP,
+		Src:     sip,
+		Dst:     dip,
+		IPID:    0,
+	}
+	tcpHeader := make([]byte, 20)
+	binary.BigEndian.PutUint16(tcpHeader[0:], sport)
+	binary.BigEndian.PutUint16(tcpHeader[2:], dport)
+	tcpHeader[13] |= 2 // SYN
+
+	both := packet.Generate(ipHeader, tcpHeader)
+
+	// 20 byte IP4 + 20 byte TCP
+	binary.BigEndian.PutUint16(both[2:4], 40)
+
+	return both
+}
+
 func nets(nets ...string) (ret []netaddr.IPPrefix) {
 	for _, s := range nets {
 		if i := strings.IndexByte(s, '/'); i == -1 {
@@ -385,3 +414,70 @@ func TestAtomic64Alignment(t *testing.T) {
 	c := new(Wrapper)
 	atomic.StoreInt64(&c.lastActivityAtomic, 123)
 }
+
+func TestPeerAPIBypass(t *testing.T) {
+	wrapperWithPeerAPI := &Wrapper{
+		PeerAPIPort: func(ip netaddr.IP) (port uint16, ok bool) {
+			if ip == netaddr.MustParseIP("100.64.1.2") {
+				return 60000, true
+			}
+			return
+		},
+	}
+
+	tests := []struct {
+		name   string
+		w      *Wrapper
+		filter *filter.Filter
+		pkt    []byte
+		want   filter.Response
+	}{
+		{
+			name: "reject_nil_filter",
+			w: &Wrapper{
+				PeerAPIPort: func(netaddr.IP) (port uint16, ok bool) {
+					return 60000, true
+				},
+			},
+			pkt:  tcp4syn("1.2.3.4", "100.64.1.2", 1234, 60000),
+			want: filter.Drop,
+		},
+		{
+			name:   "reject_with_filter",
+			w:      &Wrapper{},
+			filter: filter.NewAllowNone(logger.Discard, new(netaddr.IPSet)),
+			pkt:    tcp4syn("1.2.3.4", "100.64.1.2", 1234, 60000),
+			want:   filter.Drop,
+		},
+		{
+			name:   "peerapi_bypass_filter",
+			w:      wrapperWithPeerAPI,
+			filter: filter.NewAllowNone(logger.Discard, new(netaddr.IPSet)),
+			pkt:    tcp4syn("1.2.3.4", "100.64.1.2", 1234, 60000),
+			want:   filter.Accept,
+		},
+		{
+			name:   "peerapi_dont_bypass_filter_wrong_port",
+			w:      wrapperWithPeerAPI,
+			filter: filter.NewAllowNone(logger.Discard, new(netaddr.IPSet)),
+			pkt:    tcp4syn("1.2.3.4", "100.64.1.2", 1234, 60001),
+			want:   filter.Drop,
+		},
+		{
+			name:   "peerapi_dont_bypass_filter_wrong_dst_ip",
+			w:      wrapperWithPeerAPI,
+			filter: filter.NewAllowNone(logger.Discard, new(netaddr.IPSet)),
+			pkt:    tcp4syn("1.2.3.4", "100.64.1.3", 1234, 60000),
+			want:   filter.Drop,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			tt.w.SetFilter(tt.filter)
+			tt.w.disableTSMPRejected = true
+			if got := tt.w.filterIn(tt.pkt); got != tt.want {
+				t.Errorf("got = %v; want %v", got, tt.want)
+			}
+		})
+	}
+}