ExitSecureMemory MPU region limitation (OEMiRoT)

[Hans_W]

The OEMiRoT project uses ExitSecureMemory V1.1 to enable 1 MPU region and start the application. This automatically enables the configured HDP region, so application cannot access the bootloader. The default OEMiRoT project allows for 2 tiny 64kB applications, but I need >128kB for my application, so I added support for external SPI flash for the download location and removed unneeded data areas.

My goal is to fit the bootloader in the first 64kB and have the application use the remaining 192kB. However, an MPU region located at 64kB offset cannot be larger than 64kB itself (offset multiple of size). From a security point of view the bootloader cannot enable anything outside its own scope, but given this limitation (pass a single region to be enabled) it cannot instruct the ExitSecureMemory function to allow all flash from 64kB-256kB to be accessed.

In the documentation (like AN2606) I couldn't find if the application is still allowed to enable another MPU region to allow execution above the 128k offset, or that the ExitSecureMemory function somehow disables application access to the MPU. Par. 4.7.2 of AN2606(rev 64) isn't clear to me as well, what effect does R3=0xFF have in figure 8 if both cases act the same? the "Yes" action should be something else because 0xFF is not a region.

Will the application started by ExitSecureMemory be able to enable another MPU region?

Or should I set the MPU region to be enabled equal to the whole flash (overlapping the HDP area) and relying on HDP to protect the secure bootloader?

[Jocelyn RICARD]

Hello @Hans_W,

could you please tell which STM32 you are talking about ?

Thank you

Best regards

Jocelyn

[Hans_W]

I'm using the STM32U083, to replace the less secure L071 in an existing product

[Jocelyn RICARD · Accepted solution]

Hello @Hans_W ,

Yes, there is an issue in the AN2606. If R3 is 0xFF, then no MPU region is set. I will request for documentation update.

Regarding your question, the solution would be to 

1) declare the whole 256KB in the MPU

2) use the subregion disable to disable the first 64KB. There are 8 subregions. 256/8 = 32KB. So, need to disable the first 2 subregions. Subregion value should then be 0x03.

This way you have the full active slot protected by MPU and if the firmware tries to access address located in the OEMiRoT a fault will be raised.

Best regards

Jocelyn

[Hans_W]

Hi @Jocelyn RICARD 

Thanks for the idea. I didn't find the concept of subregions yet, but this seems the way to go to allow exactly the range 64kB-256kB using a single region definition.