net/tstun: block looped disco traffic, take 17

It was in the wrong filter direction before, per CPU profiles
we now have.

Updates #1526 (maybe fixes? time will tell)

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
(cherry picked from commit 080381c79f)

net/tstun/wrap.go

@@ -413,6 +413,16 @@ func (t *Wrapper) filterOut(p *packet.Parsed) filter.Response {
 		return filter.DropSilently // don't pass on to OS; already handled
 	}
 
+	// Issue 1526 workaround: if we sent disco packets over
+	// Tailscale from ourselves, then drop them, as that shouldn't
+	// happen unless a networking stack is confused, as it seems
+	// macOS in Network Extension mode might be.
+	if p.IPProto == ipproto.UDP && // disco is over UDP; avoid isSelfDisco call for TCP/etc
+		t.isSelfDisco(p) {
+		t.logf("[unexpected] received self disco out packet over tstun; dropping")
+		return filter.DropSilently
+	}
+
 	if t.PreFilterOut != nil {
 		if res := t.PreFilterOut(p, t); res.IsDrop() {
 			return res
@@ -517,7 +527,7 @@ func (t *Wrapper) filterIn(buf []byte) filter.Response {
 	// macOS in Network Extension mode might be.
 	if p.IPProto == ipproto.UDP && // disco is over UDP; avoid isSelfDisco call for TCP/etc
 		t.isSelfDisco(p) {
-		t.logf("[unexpected] received self disco package over tstun; dropping")
+		t.logf("[unexpected] received self disco in packet over tstun; dropping")
 		return filter.DropSilently
 	}
 

net/tstun/wrap_test.go

@@ -513,7 +513,18 @@ func TestFilterDiscoLoop(t *testing.T) {
 	if got != filter.DropSilently {
 		t.Errorf("got %v; want DropSilently", got)
 	}
-	if got, want := memLog.String(), "[unexpected] received self disco package over tstun; dropping\n"; got != want {
+	if got, want := memLog.String(), "[unexpected] received self disco in packet over tstun; dropping\n"; got != want {
+		t.Errorf("log output mismatch\n got: %q\nwant: %q\n", got, want)
+	}
+
+	memLog.Reset()
+	pp := new(packet.Parsed)
+	pp.Decode(pkt)
+	got = tw.filterOut(pp)
+	if got != filter.DropSilently {
+		t.Errorf("got %v; want DropSilently", got)
+	}
+	if got, want := memLog.String(), "[unexpected] received self disco out packet over tstun; dropping\n"; got != want {
 		t.Errorf("log output mismatch\n got: %q\nwant: %q\n", got, want)
 	}
 }