Cmep

Cmep, also named Cry, is a processor in the PS Vita responsible for most of the cryptography tasks including storing and handing of the most secure keys. Cmep was named "F00D" by Team Molecule after the  field of the ELF headers, but its official name is CMeP (standing for either Crypto MeP or Control MeP). Cmep is a Toshiba MeP-c5 core.

More information can be found in Toshiba documentations. For example: https://toshiba.semicon-storage.com/info/docget.jsp?did=14997&prodName=TMPV7504XBG.

Devices
See Physical_Memory and Cmep registers for physical memory mapping.

Reset
Although the MeP architecture documentations specify that with EVM=0 the reset/NMI vector base is at 0x00000000, it is observed that the vector base is actually at 0x00040000. However, EVA/IVA still works as expected when EVM=1. Both Secure Kernel and Second Loader set EVM=0 at the start. This is likely a modified hardware behavior and the vector base remapping might be done when the First Loader is unmapped.

Protocol
A 32-bit command buffer is defined below. The command is sent to Cmep with the method listed above.

Command ID
There are a total of 17 commands. Commands pass data through the 0x100 shared buffer that is setup in command 0.

Command 0
This command is used to setup a shared buffer with Cmep. Firstly, Cmep will write in  either 0x200 or 0x600 (and if an error happened, 0x400?). If the value 0x600 is written, it will setup a buffer that contains the secure_kernel.enp. It will write 1 to  and after that 0x0 to , next, it will wait   return 0 and   return a value <= 0x0. If everything goes alright, it will clear the low 2 bits of the secure_kernel.enp address (normally at 0x1F850000) and OR it with 0x1 and then write it to. After it, the process is the same that the 0x200. If the value 0x200 is written, it will set the 0x100-sized shared buffer. The physical address of the buffer is written to  and then command 0x0 is written.

Command 1
This command is called by a subroutine of SceSblSmsched or SceSblSmsched module_start (if an error happened while SceSblSmsched was initializing). This command seems to be used to ask Cmep to remove all allocated resources (like shared memory address, loaded Secure Module, etc), and probably power off or reset it.

Command 2
Load a Secure Module.

Command 3
Load the previously suspended Secure Module.

Command 4
Suspend the current Secure Module.

Command 5
Kill the current Secure Module.

Command 6
Force stop? Called from sceSblSmSchedDeleteAll.

Command 7
Unused.

Command 8
Unused.

Command 9
Set a 0x80 sized shared buffer.

This seems like a SceKernelPARange.

Command 10
Set the encrypted signed revoke list.

Interrupts
There are 4 interrupts that Cmep sends. There are only two interrupt handles, however.


 * : handles interrupt 200. This is notifications about current Secure Module status changes (loaded, unloaded, suspended, etc.)
 * : handles interrupts 201, 202, 203. This interrupt is sent once an Secure Module function has finished processing (see below).

Executing Secure Module functions
You can find an open source ARM TrustZone<->Cmep protocol implementation at https://github.com/xyzz/f00d. It allows calling Secure Modules commands with a maximum of control over the sent data, received data and timing.

When a Secure Module is loaded, you can execute functions it provides by writing into registers 0xE0000014, 0xE0000018, 0xE000001C the physical address of the request buffer OR'd with 1.

Once result is available, ARM will get an interrupt 201-203. Read status code from 0xE0000004 or 0xE0000008 or 0xE000000C and confirm you received it by writing it back to the same register.

Possible results:


 * 1: command executed successfully
 * 3: invalid function ID
 * 5: invalid function buffer physical address (e.g. trying to pass secure memory)
 * 9: SCE_SBL_ERROR_COMMON_EIO

Memory
Cmep has its own private 128kB memory from  to. Secure Kernel is loaded to. Secure Modules are typically loaded to.

Secure Modules Task states
ARM TrustZone implements a scheduler for Secure Modules tasks. Normally, only one task can run on Cmep at a time. The external Secure Modules scheduler running on ARM allows to have multiple tasks at once. The following provides an explanation of various states the task can be in.

Related pages

 * Cmep basics
 * Cmep registers
 * F00D Key Ring Base
 * Secure Block
 * First Loader
 * Second Loader
 * Secure Kernel
 * Secure Modules
 * Secure Modules Functions
 * Ernie Secure