Exploration of ARM TrustZone Technology

ARM TrustZone technology has been around for almost a decade. It was introduced at a time when the controversial discussion about trusted platform-modules (TPM) on x86 platforms was in full swing (TCPA, Palladium). Similar to how TPM chips were meant to magically make PCs "trustworthy", TrustZone aimed at establishing trust in ARM-based platforms. In contrast to TPMs, which were designed as fixed-function devices with a predefined feature set, TrustZone represented a much more flexible approach by leveraging the CPU as a freely programmable trusted platform module. To do that, ARM introduced a special CPU mode called "secure mode" in addition to the regular normal mode, thereby establishing the notions of a "secure world" and a "normal world". The distinction between both worlds is completely orthogonal to the normal ring protection between user-level and kernel-level code and hidden from the operating system running in the normal world. Furthermore, it is not limited to the CPU but propagated over the system bus to peripheral devices and memory controllers. This way, such an ARM-based platform effectively becomes a kind of split personality. When secure mode is active, the software running on the CPU has a different view on the whole system than software running in non-secure mode. This way, system functions, in particular security functions and cryptographic credentials, can be hidden from the normal world. It goes without saying that this concept is vastly more flexible than TPM chips because the functionality of the secure world is defined by system software instead of being hard-wired

https://genode.org/documentation/articles/trustzone