SA and SAU

Preface

  SA和SAU是由Armv8-M的security extension引入的。是Cortex-M系列中实现TZ的重要组成部分。本文对它们做个初步介绍。

SA

  关于SA，简单点讲就是把memory分成了下面几种type：

• Non-secure 在Arm Cortex-A系列里，non-secure的resource是其他所有状态下都可以访问的。然而在Coretex-M里并不是，在设计的时候一般都会给物理内存两段映射地址，一段供secure state访问，如[0x10000000 - 0x10040000]，一段给non-secure state访问，如[0x00000000 - 0x00040000]。在SAU设置某段物理内存为non-secure就是设置non-secure的地址区间，如[0x00003000 - 0x00004000]，这段地址secure state是不能访问的，但可以通过secure的地址映射[0x10003000 - 0x10004000]来访问。
• Secure and Non-secure callable 只有secure processor可以访问，允许non-secure world在这个区域内调用函数，当然要通过SG指令。
• Secure and not Non-secure callable 只有secure processor可以访问。

  另外还有exception。

0xF0000000 - 0xFFFFFFFF
If the PE implements the Security Extension, this memory range is always marked as Secure and not Non-secure callable for instruction fetches.
If the Security Extension is not present, this range is marked as Non-secure.

  以及下面的地址由当前processor的security state决定。

The following address ranges are marked with the Security state indicated by NS-Req, that is, the current state of the PE for non-debug accesses. This marking sets the NS-Attr to NS-Req:
0xE0000000 - 0xE0002FFF: ITM, DWT, FPB.
0xE000E000 - 0xE000EFFF: SCS range.
0xE002E000 - 0xE002EFFF: SCS NS alias range.
0xE0040000 - 0xE0041FFF: TPIU, ETM.
0xE00FF000 - 0xE00FFFFF: ROM table.
0xE0000000 - 0xEFFFFFFF for instruction fetch only.
Additional address ranges specified by the IDAU.

  Note: NS-Req defines the Security state that the PE or DAP requests that a memory access is performed in. 对应关系如下：

  下图是“Security attribution and MPU check sequence”。

SAU

  S先列下几个关于SAU的rule：

• RMPJC Memory is marked as Secure by default. However, if the address matches a region with SAU_REGIONn.ENABLE set to 1 and SAU_REGIONn_NSC set to 0, then memory is marked as Non-secure。
• RWGDK An address that matches multiple SAU regions is marked as Secure and not Not-secure callable regardless of the attributes specified by the regions that matched the address。

SAU Registers

  SAU的寄存器有点类似MPU的，当然功能是不一样的。

SAU_TYPE

• SREGION, bits [7:0] 读取该寄存器获取可设置的SAU region的数目。如果是0，则SAU一切都没有了意义。

SAU_RNR

• REGION, bits [7:0] 选定要配置的region的index。范围是从0到SREGION - 1。

SAU_CTRL

• ALLNS, bit [1]
  • 0 Memory is marked as Secure and is not Non-secure callable
  • 1 Memory is marked as Non-secure
• ENABLE, bit [0] 是否enable SAU
  这里需要说明的是，如果disable SAU，并且系统中也没有IDAU，那么reset回来系统默认的Secure state是无法改变的，也就是无法切换到non-secure state下。

SAU_RBAR

• BADDR, bits [31:5] 设置RNR寄存器选定region的及地址，同MPU一样，32B对齐。

SAU_RLAR

• LADDR, bits [31:5] 设置当前选定region的范围。32B对齐。
• NSC, bit [1]
  • 0 Region is not Non-secure callable 这里貌似不够明显，查看了一些资料和代码，设为0则该region是non-secure。
  • 1 Region is Non-secure callable （secure，non-secure callable）
• ENABLE, bit [0]
  • 0 SAU region is disabled
  • 1 SAU region is enabled

  另外还有两个寄存器SFSR和SFAR用来出错时debug用。

• SFSR Provides information about any security related faults
• SFAR Shows the address of the memory location that caused a Security violation

Application

  系统启动后一般SAU默认是disable的，这样在切换到non-secure的时候要做相应设置。如前所述，一定要设为enable才能切换到non-secure。另外为了安全性，应该默认所有的memory为secure。这样SAU_CTRL的设置为：

SAU_CTRL.ALLNS = 0;
SAU_CTRL.ENABLE = 1;

  为了让non-secure OS能够运行，要通过SAU设置至少一块non-secure的memory。例如：

SAU_RNR.REGION = 0;
SAU_RBAR.BADDR = 0x200000;
SAU_RLAR.LADDR = 0x400000;
SAU_RLAR.NSC = 0;
SAU_RLAR.ENABLE = 1;

  当然还要设一个secure，callable的region供non-secure world调用secure world提供的API。

SAU_RNR.REGION = 1;
SAU_RBAR.BADDR = 0x400000;
SAU_RLAR.LADDR = 0x500000;
SAU_RLAR.NSC = 1;
SAU_RLAR.ENABLE = 1;

  最后，关于IDAU由于是implementation defined，遇到具体实例再补充。

Reference

Armv8-M Architecture Reference Manual
TrustZone technology for the ARMv8-M architecture Version 2.0
Memory system and memory partitioning