ipn, wgengine/filter: fix Shields Up recent regression and old bug

Fixes #1192 (regression)
Fixes #1193 (old bug)

ipn/local.go

@@ -562,12 +562,13 @@ func (b *LocalBackend) updateFilter(netMap *controlclient.NetworkMap, prefs *Pre
 
 	localNets := unmapIPPrefixes(netMap.Addresses, advRoutes)
 
+	oldFilter := b.e.GetFilter()
 	if shieldsUp {
 		b.logf("netmap packet filter: (shields up)")
-		b.e.SetFilter(filter.NewShieldsUpFilter(b.logf))
+		b.e.SetFilter(filter.NewShieldsUpFilter(localNets, oldFilter, b.logf))
 	} else {
 		b.logf("netmap packet filter: %v", packetFilter)
-		b.e.SetFilter(filter.New(packetFilter, localNets, b.e.GetFilter(), b.logf))
+		b.e.SetFilter(filter.New(packetFilter, localNets, oldFilter, b.logf))
 	}
 }
 

wgengine/filter/filter.go

@@ -132,8 +132,16 @@ func NewAllowNone(logf logger.Logf) *Filter {
 	return New(nil, nil, nil, logf)
 }
 
-func NewShieldsUpFilter(logf logger.Logf) *Filter {
+// NewShieldsUpFilter returns a packet filter that rejects incoming connections.
-	f := New(nil, nil, nil, logf)
+//
+// If shareStateWith is non-nil, the returned filter shares state with the previous one,
+// as long as the previous one was also a shields up filter.
+func NewShieldsUpFilter(localNets []netaddr.IPPrefix, shareStateWith *Filter, logf logger.Logf) *Filter {
+	// Don't permit sharing state with a prior filter that wasn't a shields-up filter.
+	if shareStateWith != nil && !shareStateWith.shieldsUp {
+		shareStateWith = nil
+	}
+	f := New(nil, localNets, shareStateWith, logf)
 	f.shieldsUp = true
 	return f
 }