tree d80efd2c15847d619ceba36095c449f765a72631
parent ca099b9e9bec6178917fe41cba12cda75be56e29
author Gaurav Aggarwal <aggarg@amazon.com> 1662542234 +0530
committer Gaurav-Aggarwal-AWS <33462878+aggarg@users.noreply.github.com> 1663353188 +0530

Use highest numbered MPU regions for kernel

ARMv7-M allows overlapping MPU regions. When 2 MPU regions overlap, the
MPU configuration of the higher numbered MPU region is applied. For
example, if a memory area is covered by 2 MPU regions 0 and 1, the
memory permissions for MPU region 1 are applied.

We use 5 MPU regions for kernel code and kernel data protections and
leave the remaining for the application writer. We were using lowest
numbered MPU regions (0-4) for kernel protections and leaving the
remaining for the application writer. The application writer could
configure those higher numbered MPU regions to override kernel
protections.

This commit changes the code to use highest numbered MPU regions for
kernel protections and leave the remaining for the application writer.
This ensures that the application writer cannot override kernel
protections.

We thank the SecLab team at Northeastern University for reporting this
issue.

Signed-off-by: Gaurav Aggarwal <aggarg@amazon.com>