ARM TrustZone

Introduction

ARM’s TrustZone technology aims to enhance security by providing a safe execution environment for sensitive code, separated from the standard “Normal World.” TrustZone achieves this by dividing system resources into secure and non-secure partitions.

• Secure World: A highly privileged environment where sensitive code like cryptographic algorithms and key storage run.
• Normal World: The general-purpose, less-privileged environment where the operating system and user applications operate.

Switching Between Worlds

Switching between the secure and normal worlds is done through a “world switch,” facilitated by specific instructions and often controlled by the monitor mode.

Exception Levels (ELs)

ARM CPUs have Exception Levels (ELs) from EL0 to EL3, with EL3 being the most privileged. ELs are crucial in understanding security boundaries:

• EL3: Highly privileged, usually the first code to execute during the boot process.
• EL2: Used for virtualization and also has its security implications.
• EL1: The OS Kernel often runs here.
• EL0: User-level applications.

You can switch between these levels by taking an exception/interrupt or returning from one.

For instance, ARM’s eret instruction can be used to switch ELs even in the absence of an actual interrupt.

AArch64 Internals

AArch64 is the 64-bit state introduced in the ARMv8-A architecture.

The AArch64 architecture includes features that allow better performance, including more general-purpose registers and improved exception handling.

• System Registers: Such as CurrentEL to identify the current EL.
• Instructions: Such as mrs to read system registers and msr to write to them.

Attack Vectors

• Secure API Exploits: Secure APIs are often used for communication between worlds. Incomplete validation or bugs can lead to critical vulnerabilities.
• World Switching: Vulnerabilities might exist in the world switch mechanism, allowing an adversary to execute code in the secure world from the normal world.
• Exception Level Elevation: Errors in switching or managing ELs can lead to privilege escalation. Monitoring instructions like eret can be critical here.
• Secure Boot Flaws: Since EL3 is the first to execute, vulnerabilities here can compromise the entire system.
• Trusted Applications (TAs): These are essentially secure world software components. Vulnerabilities in TAs can compromise secure data.

Tools and Techniques

• Reverse Engineering Tools: IDA Pro, Ghidra for analyzing secure world binaries.
• Virtualization: QEMU for simulating TrustZone environments.
• Debuggers: JTAG for low-level debugging.
• Static Analysis Tools: For analyzing API calls, data flows, etc.

Further Study Areas

• ARM Architecture Reference Manuals: To understand AArch64 instructions and features.
• TrustZone API Documentation: To delve deeper into secure APIs.
• CVE Databases: To understand past vulnerabilities in TrustZone and related components.