how to cast void* to shared_ptr<mytype>

Since you are storing a pointer to an instance of std::shared_ptr you need to cast the value returned by getUserPointer to std::shared_ptr<>* instead of just std::shared_ptr<>.

std::shared_ptr<disk_node>* u_poi
  = static_cast< std::shared_ptr<disk_node>* >(RayCallback.m_collisionObject->getUserPointer());