STM32H7 Prevent MPU re-configuration

[mwp]

Hello all,

I wish to setup the MPU early in my firmware, then prevent changes to the MPU registers until next power-cycle/reboot.

Is this possible without having the root secure services? 

The MCU im using is the STM32H7A3.

Thanks in advance.

[FBL]

Hello @mwp 

You can configure MPU without RSS. To learn more about MPU, check this Introduction to memory protection unit management on STM32 MCUs - Application note

[Pavel A.]

@mwp This is not really possible on STM32H7. You can divide your software to "privileged" and "non-privileged" modes and disable for the latter access to MPU.

[mwp]

Yes, but is there a way to prevent modifications to MPU registers after initial configuration?

[mwp]

@Pavel A. thanks for the reply. It looks like I have more studying to do.

I'm aiming to create a bootloader residing in internal flash that does not allow 3rd party firmware to have any kind of access to the bootloader's firmware. I need to protect keys in the bootloader.

[Pavel A.]

With STM32H7A3 your only option is PCROP, if I understand correctly. PCROP is basically execute-only mode, it can be used to encrypt or sign some data with a private key, which cannot be read otherwise. More details in the reference manual.

[FBL]

It is possible to lock the MPU configuration and it should not be changed in privileged mode. 

void MPU_AccessPermConfig(void)
{
  /* Configure region for PrivilegedReadOnlyArray as REGION N°3, 32byte and R
     only in privileged mode */
  /* Disable MPU */
  LL_MPU_Disable();

  LL_MPU_ConfigRegion(REGION_NUMBER, 0x00, ARRAY_ADDRESS_START, 
         REGION_SIZE | portMPU_REGION_PRIVILEGED_READ_ONLY | LL_MPU_ACCESS_NOT_BUFFERABLE |
         LL_MPU_ACCESS_NOT_CACHEABLE | LL_MPU_ACCESS_NOT_SHAREABLE | LL_MPU_TEX_LEVEL0 |
         LL_MPU_INSTRUCTION_ACCESS_ENABLE);
  
  /* Enable MPU (any access not covered by any enabled region will cause a fault) */
  LL_MPU_Enable(LL_MPU_CTRL_PRIVILEGED_DEFAULT);

  /* Read from PrivilegedReadOnlyArray. This will not generate error */
  (void)PrivilegedReadOnlyArray[ARRAY_SIZE - 1];
}

[Pavel A.]

Hi Firas, this will protect the MPU from non-privileged code - but then the bootloader must run the user's program in non-privileged mode. User won't be able to have their own interrupt handlers, as they are privileged...

[FBL]

Hi @Pavel A. 

You are right. With STM32H7B3, this would be possible since it has HW crypto.