How to Implement ARM TrustZone for Secure Firmware Updates in STM32

Implementing ARM TrustZone for Secure Firmware Updates in STM32 Microcontrollers

ARM TrustZone technology provides a way to create a secure environment within the STM32 microcontroller, enabling secure firmware updates. This guide walks you through the prerequisites, required tools, steps to implement TrustZone, and troubleshooting tips.

Prerequisites

• Basic knowledge of embedded systems and STM32 microcontrollers.
• Familiarity with ARM architecture and TrustZone technology.
• STM32 development board (e.g., STM32F7, STM32H7).
• Development environment set up (IDE such as STM32CubeIDE).
• Access to firmware source code and update images.

Parts/Tools

• STM32 development board with TrustZone support.
• STM32CubeIDE or other compatible IDE.
• STM32CubeMX for peripheral configuration.
• Secure firmware update tool (e.g., custom bootloader).
• JTAG/SWD programmer for debugging (e.g., ST-Link).

Steps

1. Set Up the Development Environment
   1. Install STM32CubeIDE from the STMicroelectronics website.
   2. Install STM32CubeMX if not already included.
   3. Set up a new project for your STM32 microcontroller.
2. Configure the Microcontroller for TrustZone
   1. Open STM32CubeMX and select your microcontroller.
   2. Enable TrustZone in the “System Core” settings.
   3. Configure peripherals as required for your application.
   4. Generate the project files and open them in STM32CubeIDE.
3. Implement the Secure and Non-Secure Code
   1. Create separate folders for secure and non-secure firmware.
   2. Implement the secure firmware functionality in the secure folder.
   3. Example secure function in C:

      void SecureFunction(void) {
                          // Secure operation code
                      }

   4. Implement the non-secure firmware functionality in the non-secure folder.
   5. Example non-secure function in C:

      void NonSecureFunction(void) {
                          // Non-secure operation code
                      }

4. Create a Bootloader for Firmware Updates
   1. Design a bootloader that runs before the main firmware.
   2. Ensure the bootloader checks for a new firmware update.
   3. Example bootloader check in C:

      if (CheckForUpdate()) {
                          UpdateFirmware();
                      }

   4. Implement the `UpdateFirmware` function to handle secure firmware updates.
5. Test the Implementation
   1. Connect your STM32 board to the development environment using a JTAG/SWD debugger.
   2. Load the secure firmware onto the device.
   3. Run the bootloader to verify firmware update functionalities.
   4. Test both secure and non-secure functionalities to ensure correct operation.

Troubleshooting

• Bootloader Fails to Start:
  • Check the boot configuration settings in STM32CubeMX.
  • Ensure that the correct memory locations are set for the bootloader.
• Secure Function Not Executing:
  • Verify that the secure function is properly defined and called.
  • Check the memory attributes to ensure the secure region is correctly set.
• Firmware Update Issues:
  • Ensure the update image is correctly formatted and signed.
  • Check for any errors in the update process and ensure that the bootloader correctly verifies the update.

Conclusion

Implementing ARM TrustZone for secure firmware updates on STM32 microcontrollers enhances the security of your applications. By following the steps outlined in this tutorial, you can effectively set up a secure environment for your firmware, ensuring that updates are performed safely and securely. Remember to test thoroughly and troubleshoot any issues that arise during the implementation.