ssh/server: handle repeated SSH authentication service requests

ssh/server.go

@@ -310,8 +310,8 @@ func (s *connection) serverHandshake(config *ServerConfig) (*Permissions, error)
 	s.sessionID = s.transport.getSessionID()
 	s.algorithms = s.transport.getAlgorithms()
 
-	var packet []byte
-	if packet, err = s.transport.readPacket(); err != nil {
+	packet, err := s.transport.readPacket()
+	if err != nil {
 		return nil, err
 	}
 
@@ -546,13 +546,36 @@ userAuthLoop:
 			return nil, &ServerAuthError{Errors: authErrs}
 		}
 
-		var userAuthReq userAuthRequestMsg
-		if packet, err := s.transport.readPacket(); err != nil {
+		packet, err := s.transport.readPacket()
+		if err != nil {
 			if err == io.EOF {
 				return nil, &ServerAuthError{Errors: authErrs}
 			}
 			return nil, err
-		} else if err = Unmarshal(packet, &userAuthReq); err != nil {
+		}
+
+		// Check if this is a service request (re-authentication)
+		if len(packet) > 0 && packet[0] == msgServiceRequest {
+			var serviceRequest serviceRequestMsg
+			if err = Unmarshal(packet, &serviceRequest); err != nil {
+				return nil, err
+			}
+
+			if serviceRequest.Service == serviceUserAuth {
+				serviceAccept := serviceAcceptMsg{
+					Service: serviceUserAuth,
+				}
+				if err := s.transport.writePacket(Marshal(&serviceAccept)); err != nil {
+					return nil, err
+				}
+				continue userAuthLoop
+			} else {
+				return nil, fmt.Errorf("ssh: unknown service %q", serviceRequest.Service)
+			}
+		}
+
+		var userAuthReq userAuthRequestMsg
+		if err = Unmarshal(packet, &userAuthReq); err != nil {
 			return nil, err
 		}