What is TrustZone?

TrustZone is the name of the Security architecture in the Arm A-profile architecture. First introduced in Armv6K, TrustZone is also supported in Armv7-A and Armv8-A. TrustZone provides two execution environments with system-wide hardware enforced isolation between them, as shown in this diagram:



The Normal world runs a rich software stack. This software stack typically includes a large application set, a complex operating system like Linux, and possibly a hypervisor. Such software stacks are large and complex. While efforts can be made to secure them, the size of the attack surface means that they are more vulnerable to attack.

The Trusted world runs a smaller and simpler software stack, which is referred to as a Trusted Execution Environment (TEE). Typically, a TEE includes several Trusted services that are hosted by a lightweight kernel. The Trusted services provide functionality like key management. This software stack has a considerably smaller attack surface, which helps reduce vulnerability to attack.

Note: You might sometimes see the term Rich Execution Environment (REE) used to describe the software that is running in the Normal world.

TrustZone aims to square a circle. As users and developers, we want the rich feature set and flexibility of the Normal world. At the same time, we want the higher degrees of trust that it is possible to achieve with a smaller and more restricted software stack in the Trusted world. TrustZone gives us both, providing two environments with hardware-enforced isolation between them.

TrustZone for Armv8-M

TrustZone is also used to refer the Security Extensions in the Armv8-M architecture. While there are similarities between TrustZone in the A profile architecture and the M profile architecture, there are also important differences. This guide covers the A profile only.

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