.. zephyr:code-sample:: mpu
:name: Memory Protection Unit (MPU)
Use memory protection to prevent common security issues.
Overview
********
This application provides a set options to check the correct MPU configuration
against the following security issues:
* Read at an address that is reserved in the memory map.
* Write into the boot Flash/ROM.
* Run code located in SRAM.
If the MPU configuration is correct each option selected ends up in an MPU
fault.
Building and Running
********************
This project can be built and executed as follows:
.. zephyr-app-commands::
:zephyr-app: samples/arch/mpu/mpu_test
:board: v2m_beetle
:goals: build flash
:compact:
To build the single thread version, use the supplied configuration file for
single thread: :file:`prj_single.conf`:
.. zephyr-app-commands::
:zephyr-app: samples/arch/mpu/mpu_test
:board: v2m_beetle
:conf: prj_single.conf
:goals: run
:compact:
To build a version that allows writes to the flash device, edit
``prj.conf``, and follow the directions in the comments to enable the
proper configs.
Sample Output
=============
When running the above on an ARMv7 or ARMv8 CPU, the output of each command may look
like the following.
.. code-block:: console
uart:~$ mpu read
<err> os: ***** BUS FAULT *****
<err> os: Precise data bus error
<err> os: BFAR Address: 0x24000000
<err> os: r0/a1: 0x00009a5c r1/a2: 0x00000008 r2/a3: 0x20001aa8
<err> os: r3/a4: 0x24000000 r12/ip: 0x00000000 r14/lr: 0x000029b7
<err> os: xpsr: 0x21000000
<err> os: Faulting instruction address (r15/pc): 0x000003c8
<err> os: >>> ZEPHYR FATAL ERROR 0: CPU exception on CPU 0
<err> os: Current thread: 0x20000708 (shell_uart)
<err> os: Halting system
.. code-block:: console
uart:~$ mpu write
write address: 0x4000
<err> os: ***** MPU FAULT *****
<err> os: Data Access Violation
<err> os: MMFAR Address: 0x4000
<err> os: r0/a1: 0x00000000 r1/a2: 0x0000000e r2/a3: 0x0badc0de
<err> os: r3/a4: 0x00004000 r12/ip: 0x00000004 r14/lr: 0x000003ab
<err> os: xpsr: 0x61000000
<err> os: Faulting instruction address (r15/pc): 0x000003b2
<err> os: >>> ZEPHYR FATAL ERROR 0: CPU exception on CPU 0
<err> os: Current thread: 0x20000708 (shell_uart)
<err> os: Halting system
.. code-block:: console
uart:~$ mpu run
<err> os: ***** MPU FAULT *****
<err> os: Instruction Access Violation
<err> os: r0/a1: 0x00009a5c r1/a2: 0x00000001 r2/a3: 0x20001aa8
<err> os: r3/a4: 0x20000000 r12/ip: 0x00000000 r14/lr: 0x00006673
<err> os: xpsr: 0x60000000
<err> os: Faulting instruction address (r15/pc): 0x20000000
<err> os: >>> ZEPHYR FATAL ERROR 0: CPU exception on CPU 0
<err> os: Current thread: 0x20000708 (shell_uart)
<err> os: Halting system
When running this test on an ARMv6 CPU (specifically on a Cortex-M0+), the output will
look different, as ARMv6 handles MPU faults as generic hard faults. Moreover, when
running the ``mpu run`` command, Zephyr's hard fault handler for AArch32 tries to
dereference the program counter from the exception stack frame, causing a second fault
and silently placing the processor into a lockup state.
To verify that the ``mpu run`` test in particular is running correctly, one can
* start the MPU test under gdb with ``west debug``
* execute ``mpu run`` over UART
* then interrupt gdb with Ctrl-C and show the program counter by running ``l`` in gdb.
The program counter should display as ``0xfffffffe``, indicating a lockup state.
.. code-block:: console
uart:~$ mpu read
<err> os: ***** HARD FAULT *****
<err> os: r0/a1: 0x0800a54c r1/a2: 0x00000008 r2/a3: 0x08003
<err> os: r3/a4: 0x24000000 r12/ip: 0x00000040 r14/lr: 0x0800d
<err> os: xpsr: 0x01000000
<err> os: Faulting instruction address (r15/pc): 0x08000486
<err> os: >>> ZEPHYR FATAL ERROR 0: CPU exception on CPU 0
<err> os: Current thread: 0x200006a8 (shell_uart)
<err> os: Halting system
.. code-block:: console
uart:~$ mpu write
<err> os: ***** HARD FAULT *****
<err> os: r0/a1: 0x00000000 r1/a2: 0x0000000e r2/a3: 0x0000e
<err> os: r3/a4: 0x0badc0de r12/ip: 0x00000000 r14/lr: 0x08009
<err> os: xpsr: 0x61000000
<err> os: Faulting instruction address (r15/pc): 0x0800046a
<err> os: >>> ZEPHYR FATAL ERROR 0: CPU exception on CPU 0
<err> os: Current thread: 0x200006a8 (shell_uart)
<err> os: Halting system
.. code-block:: console
uart~$ mpu run
<no output>
50964e4ef9