What are the best practices for using ARMv8-M security extensions?

1 Understand the security model

The security extensions of ARMv8-M are based on the concept of TrustZone, which divides the system into two security states: secure and non-secure. The secure state has access to all the resources of the system, while the non-secure state has limited access, and can only access the secure state through predefined entry points. The security state is controlled by the hardware, and can be switched by specific instructions or exceptions. To use the security extensions, you need to understand how the security state affects the memory, peripherals, interrupts, and exceptions of the system, and how to configure them accordingly.

4 Follow coding guidelines

To ensure the security and robustness of your code, you should adhere to certain coding guidelines. Utilizing static analysis tools can help detect potential issues like buffer overflows, memory leaks, or uninitialized variables. Additionally, a consistent coding style and naming convention should be used and documented clearly. Global variables and pointers should be avoided as they can introduce security risks or side effects. Defensive programming techniques such as input validation, error handling, and assert statements should be implemented to prevent unexpected behavior or malicious inputs. Finally, secure coding standards such as MISRA C or CERT C should be followed to comply with the best practices for safety and security.

5 Test and debug your code

Testing and debugging your code is crucial to verify its functionality and security, as well as to identify and fix any issues or defects. This can be challenging, however, due to the limited visibility and access from the non-secure state. To address this, you should consider using tools and techniques such as a debugger with security extensions, a simulator that emulates the security extensions, or a test framework that supports the security extensions. These can offer features such as breakpoints, watchpoints, trace, analysis, virtual platform control, and compliance with security standards and specifications.

6 Keep your code updated

The last best practice is to keep your code updated with the latest patches, updates, and improvements, as well as to monitor and respond to any security incidents or vulnerabilities that may affect your system. This can help maintain security and performance, as well as meet regulatory and industry requirements. To do this, you can use a version control system such as Git or SVN to manage your code and track its changes and history. Additionally, you can use a secure firmware update mechanism like ARM Firmware Framework for Mbed OS or TF-M Firmware Update Service to deliver and install updates securely. Finally, you can use a vulnerability management system like ARM PSA Certified Vulnerability Reporting Service or the CVE database to notify of any known or reported vulnerabilities that may affect your system and provide guidance on how to mitigate them.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?