[client] Fix daemon lock order inversion between SetConfig and login (#6978)

client/server/server.go

@@ -393,6 +393,16 @@ func (s *Server) loginAttempt(ctx context.Context, setupKey, jwtToken string) (i
 
 // Login uses setup key to prepare configuration for the daemon.
 func (s *Server) SetConfig(callerCtx context.Context, msg *proto.SetConfigRequest) (*proto.SetConfigResponse, error) {
+	// Privilege gate: refuse the parts of the request that would let a local
+	// user turn the root daemon into a root shell. Held across the write so the
+	// config cannot gain the SSH server between the decision and the update.
+	//
+	// Taken before s.mutex: authorizeAndPrepareLogin takes s.mutex while holding
+	// guardedConfigMu, so acquiring the two in the other order here would let a
+	// concurrent login deadlock the daemon.
+	s.guardedConfigMu.Lock()
+	defer s.guardedConfigMu.Unlock()
+
 	s.mutex.Lock()
 	defer s.mutex.Unlock()
 
@@ -417,12 +427,6 @@ func (s *Server) SetConfig(callerCtx context.Context, msg *proto.SetConfigReques
 		return nil, err
 	}
 
-	// Privilege gate: refuse the parts of the request that would let a local
-	// user turn the root daemon into a root shell. Held across the write so the
-	// config cannot gain the SSH server between the decision and the update.
-	s.guardedConfigMu.Lock()
-	defer s.guardedConfigMu.Unlock()
-
 	stored, err := s.storedProfileConfig(msg.ProfileName, msg.Username)
 	if err != nil {
 		return nil, err