Memory Protection Unit (MPU) Enhancements (#705)

Memory Protection Unit (MPU) Enhancements (#705)

Memory Protection Unit (MPU) Enhancements

This commit introduces a new MPU wrapper that places additional
restrictions on unprivileged tasks. The following is the list of changes
introduced with the new MPU wrapper:

1. Opaque and indirectly verifiable integers for kernel object handles:
   All the kernel object handles (for example, queue handles) are now
   opaque integers. Previously object handles were raw pointers.

2. Saving the task context in Task Control Block (TCB): When a task is
   swapped out by the scheduler, the task's context is now saved in its
   TCB. Previously the task's context was saved on its stack.

3. Execute system calls on a separate privileged only stack: FreeRTOS
   system calls, which execute with elevated privilege, now use a
   separate privileged only stack. Previously system calls used the
   calling task's stack. The application writer can control the size of
   the system call stack using new configSYSTEM_CALL_STACK_SIZE config
   macro.

4. Memory bounds checks: FreeRTOS system calls which accept a pointer
   and de-reference it, now verify that the calling task has required
   permissions to access the memory location referenced by the pointer.

5. System call restrictions: The following system calls are no longer
   available to unprivileged tasks:
    - vQueueDelete
    - xQueueCreateMutex
    - xQueueCreateMutexStatic
    - xQueueCreateCountingSemaphore
    - xQueueCreateCountingSemaphoreStatic
    - xQueueGenericCreate
    - xQueueGenericCreateStatic
    - xQueueCreateSet
    - xQueueRemoveFromSet
    - xQueueGenericReset
    - xTaskCreate
    - xTaskCreateStatic
    - vTaskDelete
    - vTaskPrioritySet
    - vTaskSuspendAll
    - xTaskResumeAll
    - xTaskGetHandle
    - xTaskCallApplicationTaskHook
    - vTaskList
    - vTaskGetRunTimeStats
    - xTaskCatchUpTicks
    - xEventGroupCreate
    - xEventGroupCreateStatic
    - vEventGroupDelete
    - xStreamBufferGenericCreate
    - xStreamBufferGenericCreateStatic
    - vStreamBufferDelete
    - xStreamBufferReset
   Also, an unprivileged task can no longer use vTaskSuspend to suspend
   any task other than itself.

We thank the following people for their inputs in these enhancements:
- David Reiss of Meta Platforms, Inc.
- Lan Luo, Xinhui Shao, Yumeng Wei, Zixia Liu, Huaiyu Yan and Zhen Ling
  of School of Computer Science and Engineering, Southeast University,
  China.
- Xinwen Fu of Department of Computer Science, University of
  Massachusetts Lowell, USA.
- Yuequi Chen, Zicheng Wang, Minghao Lin of University of Colorado
  Boulder, USA.