Month: June 2017

NVidia Tiny Linux Kernel and TrustZone

The NVidia Tiny Linux Kernel (TLK), is 23K lines of BSD licensed code, which supports multi-threading, IPC and thread scheduling and implements TrustZone features of a Trusted Execution Environment (TEE). It is based on the Little Kernel for embedded devices.

The TEE is an isolated environment that runs in parallel with an operating system, providing security. It is more secure than an OS and offers a higher level of functionality than a SE, using a hybrid approach that utilizes both hardware and software to protect data. Trusted applications running in a TEE have access to the full power of a device’s main processor and memory, while hardware isolation protects these from user installed apps running in a main operating system. Software and cryptographic isolation inside the TEE protect the trusted applications contained within from each other.

TrustZone was developed by Trusted Foundations Software which was acquired by Gemalto. Giesecke & Devrient developed a rival implementation named Mobicore. In April 2012 ARM, Gemalto and Giesecke & Devrient combined their TrustZone portfolios into a joint venture Trustonic, which was the first to qualify a GlobalPlatform-compliant TEE product in 2013.

A comparison with other hardware based security technologies is found here. Whereas a TPM is exclusively for security functions and does not have access to the CPU,  the TEE does have such access.

Attacks against TrustZone on Android are described in this blackhat talk. With a TEE exploit,  “avc_has_perm” can be modified to bypass SELinux for Android. By the way, Access Vectors in SELinux are described in this wonderful link. “avc_has_perm” is a function to check the AccessVectors allows permission.

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