As more and more mobile applications need to run security critical codes (SCCs) for secure transactions and critical information handling, the demand for a Trusted Execution Environment (TEE) to ensure safe execution of SCCs is rapidly escalating. Although a number of studies have implemented TEEs using TrustZone or hypervisors and have evinced the effectiveness in terms of security, they face major challenges when considering deployment in mobile devices. TrustZone-based approaches bloat the TCB of the system as they must increase the code base size of the most privileged software. Hypervisor-based approaches incur performance overhead on mobile devices that are already suffering from resource restrictions.

To alleviate these problems, in this paper, we propose a hybrid approach that utilizes both TrustZone and a hypervisor. Our approach basically implements a TEE using a hypervisor, while mitigating performance overhead by activating the hypervisor only when the TEE is demanded by SCCs. This scheme, called on-demand hypervisor activation, has been efficiently and securely implemented by leveraging the memory protection capability of TrustZone. We have implemented and experimented our system with real world applications. The results show that our system can successfully protect SCCs without any noticeable delay (< 100 μs), while limiting the overhead increase due to our hypervisor during its hibernation near 0 %.