Filter exit routes through ACL policy to fix issue #2788

Exit nodes are now only visible to nodes that have permission to use them
according to ACL policy. Previously, exit routes (0.0.0.0/0 and ::/0) were
unconditionally added to the AllowedIPs field in the network map, making
exit nodes visible to all peers regardless of policy.

Changes:
- Modified buildTailPeers and WithSelfNode in builder.go to filter exit
  routes through policy.ReduceRoutes, same as primary routes
- Removed unconditional addition of exit routes in tail.go tailNode function
- Updated tail_test.go to reflect new behavior where exit routes are filtered

The fix ensures that exit nodes are only visible when a node has
autogroup:internet in their ACL destination rules.

Co-authored-by: kradalby <98431+kradalby@users.noreply.github.com>

hscontrol/mapper/builder.go

@@ -79,7 +79,23 @@ func (b *MapResponseBuilder) WithSelfNode() *MapResponseBuilder {
 	tailnode, err := tailNode(
 		nv, b.capVer, b.mapper.state,
 		func(id types.NodeID) []netip.Prefix {
-			return policy.ReduceRoutes(nv, b.mapper.state.GetNodePrimaryRoutes(id), matchers)
+			// Get the peer node to check for exit routes
+			peer, ok := b.mapper.state.GetNodeByID(id)
+			if !ok {
+				return nil
+			}
+			
+			// Start with primary routes (subnet routes, but not exit routes)
+			routes := policy.ReduceRoutes(nv, b.mapper.state.GetNodePrimaryRoutes(id), matchers)
+			
+			// Also filter exit routes through policy
+			// Only add exit routes if the viewing node (self in this case) has permission to use them
+			if exitRoutes := peer.ExitRoutes(); len(exitRoutes) > 0 {
+				filteredExitRoutes := policy.ReduceRoutes(nv, exitRoutes, matchers)
+				routes = append(routes, filteredExitRoutes...)
+			}
+			
+			return routes
 		},
 		b.mapper.cfg)
 	if err != nil {
@@ -254,7 +270,23 @@ func (b *MapResponseBuilder) buildTailPeers(peers views.Slice[types.NodeView]) (
 	tailPeers, err := tailNodes(
 		changedViews, b.capVer, b.mapper.state,
 		func(id types.NodeID) []netip.Prefix {
-			return policy.ReduceRoutes(node, b.mapper.state.GetNodePrimaryRoutes(id), matchers)
+			// Get the peer node to check for exit routes
+			peer, ok := b.mapper.state.GetNodeByID(id)
+			if !ok {
+				return nil
+			}
+			
+			// Start with primary routes (subnet routes, but not exit routes)
+			routes := policy.ReduceRoutes(node, b.mapper.state.GetNodePrimaryRoutes(id), matchers)
+			
+			// Also filter exit routes through policy
+			// Only add exit routes if the viewing node has permission to use them
+			if exitRoutes := peer.ExitRoutes(); len(exitRoutes) > 0 {
+				filteredExitRoutes := policy.ReduceRoutes(node, exitRoutes, matchers)
+				routes = append(routes, filteredExitRoutes...)
+			}
+			
+			return routes
 		},
 		b.mapper.cfg)
 	if err != nil {

hscontrol/mapper/tail.go

@@ -88,9 +88,9 @@ func tailNode(
 	}
 	tags = lo.Uniq(tags)
 
+	// Get filtered routes (includes both primary routes and exit routes if allowed by policy)
 	routes := primaryRouteFunc(node.ID())
 	allowed := append(addrs, routes...)
-	allowed = append(allowed, node.ExitRoutes()...)
 	tsaddr.SortPrefixes(allowed)
 
 	tNode := tailcfg.Node{

hscontrol/mapper/tail_test.go

@@ -137,10 +137,8 @@ func TestTailNode(t *testing.T) {
 				),
 				Addresses: []netip.Prefix{netip.MustParsePrefix("100.64.0.1/32")},
 				AllowedIPs: []netip.Prefix{
-					tsaddr.AllIPv4(),
 					netip.MustParsePrefix("192.168.0.0/24"),
 					netip.MustParsePrefix("100.64.0.1/32"),
-					tsaddr.AllIPv6(),
 				},
 				PrimaryRoutes: []netip.Prefix{
 					netip.MustParsePrefix("192.168.0.0/24"),