ARM MPU _arch_buffer_validate allowing reads to kernel memory

[andrewboie]

Reproducible with this PR:
#8863
Run tests/kernel/mem_protect/syscalls on sam_e70_xplained.

The check to _arch_buffer_validate on a buffer defined with __kernel is not failing as it ought to be.

Not reproducible on frdm_k64f, so this seems specific to ARM MPU.

[andrewboie]

The MPU is protecting the memory if I call libc strlen on the same string, so this is definitely a problem with the validation function and not the MPU configuration itself.

[andrewboie]

Tracing through the execution, arm_core_mpu_buffer_validate correctly determines that the string falls within MPU region 1, which is SRAM_0. This is configured as REGION_RAM_ATTR, P_RW_U_NA.

The problem is, _get_region_ap() is returning 0x3 which corresponds to P_RW_U_RW.
Since just taking a strlen() faults, seems like _get_region_ap() is bugged and not returning the correct value, we ought to be getting back P_RW_U_NA, which is 0x1.

Off by one when shifting bits perhaps? The logic looks right....

[ioannisg]

@andrewboie submitted a PR that fixes this; the test you indicate is now successful after re-basing PR over #8868 .

[andrewboie]

thanks so much!!