Modern processors can offer hardware primitives that allow a process to run in isolation. These primitives implement a trusted execution environment (TEE) in which a program can run in such a way that the integrity and confidentiality of its execution are guaranteed. Intel's Software Guard eXtensions (SGX) is an example of such primitives and its isolated processes are called enclaves. These guarantees, however, can be easily undermined if the enclave has not been properly designed.

We propose a notion of an orderly enclave that splits the enclave’s behaviour into the following execution phases: entry, secure, ocall, and exit. Each of them imposes a set of restrictions that enforce a particular policy of access to untrusted memory and, in some cases, sanitisation conditions. A violation of these policies and conditions might indicate an undesired interaction with untrusted data/code or a lack of sanitisation, both of which can be harnessed to perpetrate attacks against the enclave. We also introduce Guardian: an open-source tool that uses symbolic execution to carry out the validation of an enclave against our notion of an orderly enclave; in this process, it also looks for some other typical attack primitives. We discuss how our approach can prevent and flag enclave vulnerabilities that have been identified in the literature. Moreover, we have evaluated how our approach fares in the analysis of some enclave samples. In this process, Guardian identified some previously undetected security issues in some of these samples. These were subsequently acknowledged and fixed by the corresponding maintainers.