v7M MPU is enabled too early

[nchong]

During the initialisation of the vmpu in the procedure vmpu_init_post:

1. The MPU is enabled by vmpu_arch_init() which calls vmpu_mpu_lock()
2. Box configurations are loaded and regions are programmed for box 0 by vmpu_load_box()

This means that the MPU is enabled at (1) and subsequently has regions programmed at (2). At (1), only static regions will have been programmed. In particular, higher-numbered regions will still have their reset value (UNKNOWN); hence, this is architecturally UNPREDICTABLE.

A fix is to move the call to vmpu_mpu_lock() after vmpu_load_box(). This guarantees that all RBAR/RASR registers have been written-to (by vmpu_mpu_invalidate()) before the MPU is enabled.

Found using ArchEx.

[ciarmcom]

ARM Internal Ref: IOTSEC-213