client/web: refactor authorizeRequest

Moves request authorization back into Server.serve to be run at
the start of any request. Fixes Synology unstable track bug where
client would get stuck unable to auth due to not rendering the
Synology redirect auth html on index.html load.

Updates tailscale/corp#14335

Signed-off-by: Sonia Appasamy <sonia@tailscale.com>

client/web/web.go

@@ -188,6 +188,9 @@ func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 }
 
 func (s *Server) serve(w http.ResponseWriter, r *http.Request) {
+	if ok := s.authorizeRequest(w, r); !ok {
+		return
+	}
 	if strings.HasPrefix(r.URL.Path, "/api/") {
 		// Pass API requests through to the API handler.
 		s.apiHandler.ServeHTTP(w, r)
@@ -199,31 +202,57 @@ func (s *Server) serve(w http.ResponseWriter, r *http.Request) {
 	s.assetsHandler.ServeHTTP(w, r)
 }
 
-// authorizePlatformRequest reports whether the request from the web client
-// is authorized to access the client for those platforms that support it.
+// authorizeRequest reports whether the request from the web client
+// is authorized to be completed.
 // It reports true if the request is authorized, and false otherwise.
-// authorizePlatformRequest manages writing out any relevant authorization
+// authorizeRequest manages writing out any relevant authorization
 // errors to the ResponseWriter itself.
-func authorizePlatformRequest(w http.ResponseWriter, r *http.Request) (ok bool) {
-	switch distro.Get() {
-	case distro.Synology:
+func (s *Server) authorizeRequest(w http.ResponseWriter, r *http.Request) (ok bool) {
+	if s.tsDebugMode == "full" { // client using tailscale auth
+		_, err := s.lc.WhoIs(r.Context(), r.RemoteAddr)
+		switch {
+		case err != nil:
+			// All requests must be made over tailscale.
+			http.Error(w, "must access over tailscale", http.StatusUnauthorized)
+			return false
+		case r.URL.Path == "/api/data" && r.Method == httpm.GET:
+			// Readonly endpoint allowed without browser session.
+			return true
+		case r.URL.Path == "/api/auth":
+			// Endpoint for browser to request auth allowed without browser session.
+			return true
+		case strings.HasPrefix(r.URL.Path, "/api/"):
+			// All other /api/ endpoints require a valid browser session.
+			session, err := s.getTailscaleBrowserSession(r)
+			if err != nil || !session.isAuthorized() {
+				http.Error(w, "no valid session", http.StatusUnauthorized)
+				return false
+			}
+			return true
+		default:
+			// No additional auth on non-api (assets, index.html, etc).
+			return true
+		}
+	}
+	// Client using system-specific auth.
+	d := distro.Get()
+	switch {
+	case strings.HasPrefix(r.URL.Path, "/assets/"):
+		// Don't require authorization for static assets.
+		return true
+	case d == distro.Synology:
 		return authorizeSynology(w, r)
-	case distro.QNAP:
+	case d == distro.QNAP:
 		return authorizeQNAP(w, r)
+	default:
+		return true // no additional auth for this distro
 	}
-	return true
 }
 
 // serveLoginAPI serves requests for the web login client.
 // It should only be called by Server.ServeHTTP, via Server.apiHandler,
 // which protects the handler using gorilla csrf.
 func (s *Server) serveLoginAPI(w http.ResponseWriter, r *http.Request) {
-	// The login client is run directly from client plugins,
-	// so first authenticate and authorize the request for the host platform.
-	if ok := authorizePlatformRequest(w, r); !ok {
-		return
-	}
-
 	w.Header().Set("X-CSRF-Token", csrf.Token(r))
 	if r.URL.Path != "/api/data" { // only endpoint allowed for login client
 		http.Error(w, "invalid endpoint", http.StatusNotFound)
@@ -346,31 +375,14 @@ func (s *Server) serveTailscaleAuth(w http.ResponseWriter, r *http.Request) {
 // It should only be called by Server.ServeHTTP, via Server.apiHandler,
 // which protects the handler using gorilla csrf.
 func (s *Server) serveAPI(w http.ResponseWriter, r *http.Request) {
-	if s.tsDebugMode == "full" {
-		// tailscale/corp#14335: Only restrict to tailscale auth in debug "full" web client mode.
-		// TODO(sonia,will): Switch serveAPI over to always require TS auth when we're ready
-		// to remove the debug flags.
-		// For now, existing client uses platform auth (else case below).
-
-		if r.URL.Path == "/api/auth" {
-			// Serve auth, which creates a new session for the user to authenticate,
-			// in the case that the request doesn't already have one.
-			s.serveTailscaleAuth(w, r)
-			return
-		}
-		// For all other endpoints, require a valid session to proceed.
-		session, err := s.getTailscaleBrowserSession(r)
-		if err != nil || !session.isAuthorized() {
-			http.Error(w, "no valid session", http.StatusUnauthorized)
-			return
-		}
-	} else if ok := authorizePlatformRequest(w, r); !ok {
-		return
-	}
-
 	w.Header().Set("X-CSRF-Token", csrf.Token(r))
 	path := strings.TrimPrefix(r.URL.Path, "/api")
 	switch {
+	case path == "/auth":
+		if s.tsDebugMode == "full" { // behind debug flag
+			s.serveTailscaleAuth(w, r)
+			return
+		}
 	case path == "/data":
 		switch r.Method {
 		case httpm.GET: