Trusted Execution Environments (TEE) are widely deployed, especially on smartphones. A recent trend in TEE development is the transition from vendor-controlled, single-purpose TEEs to open TEEs that host Trusted Applications (TAs) from multiple sources with independent tasks. This transition is expected to create a TA ecosystem needed for providing stronger and customized security to apps and OS running in the Rich Execution Environment (REE). However, the transition also poses two security challenges: enlarged attack surface resulted from the increased complexity of TAs and TEEs; the lack of trust (or isolation) among TAs and the TEE.

In this paper, we first present a comprehensive analysis on the recent CVEs related to TEE and the need of multiple TEE scheme. We then propose TEEv, a TEE virtualization architecture that supports multiple isolated, restricted TEE instances (i.e., vTEEs) running concurrently. Relying on a tiny hypervisor (we call it TEE-visor), TEEv allows TEE instances from different vendors to run in isolation on the same smartphone and to host their own TAs. Therefore, a compromised vTEE cannot affect its peers or REE; TAs no longer have to run in untrusted/unsuitable TEEs. We have implemented TEEv on a development board and a real smartphone, which runs multiple commercial TEE instances from different vendors with very small porting effort. Our evaluation results show that TEEv can isolate vTEEs and defend all known attacks on TEE with only mild performance overhead.