With the success of voice recognition techniques, users can easily control any device in smart home environments by simply saying a voice command. Based on this idea, a new group of smart devices are designed and released, which are called voice assistant. However, the voice itself is not secure and can be attacked in many ways. To defend against various types of voice replay attacks, we present a new voice liveness detection system. The basic insight of our system is that mouth opening movements will change the space size in the ear canal, which further changes the air pressure in ear canals. In this paper, we propose solutions to detect mouth opening movements using the noisy air pressure data and match them with the voices to validate the liveness of the voice source. To evaluate the effectiveness of our system, we develop a prototype on Raspberry Pi and conduct comprehensive evaluations. Experiments with ten volunteers show that our system can accurately accept voice commands from legitimate users with an accuracy of 91.72%. Moreover, our system can effectively defend current voice assistant devices from replay attacks with an accuracy of 97.2%.