Cmep

Cmep is a processor in the PS Vita that performs most of the cryptography tasks including storing and handing of the most secure keys. Cmep was named "F00D" by Team Molecule after the e_machine field of the ELF headers, but its official name is CMeP (?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#Cmep registers 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.

Communication

Communication seems to go through some sort of FIFO register.

Write

To write, put the double word into physical address 0xE0000010. Next read 0xE0000010 until it returns 0, which indicates the data was read by Cmep.

Read

To read, get a double word from physical address 0xE0000000. If it returns 0, no data is available. Otherwise, acknowledge that the data has been read by ARM TrustZonr by putting the same data into 0xE0000000.

Extra ports

In addition to the 0xE0000000 and 0xE0000010, the communication between Cmep and ARM uses some other ports.

Port	Read	Write	Used by
0xE0000004	YES	?	SMC 0x12d, 0x136, 0x137, 0x13B Interrupt 0xC8
0xE0000008	YES	?	SMC 0x12d, 0x136, 0x137, 0x13B Interrupt 0xC8
0xE000000C	YES	?	SMC 0x12d, 0x136, 0x137, 0x13B Interrupt 0xC8
0xE0000014	YES	YES	Used to send commands to Secure Module, see Executing Secure Module functions
0xE0000018	YES	YES	Used to send commands to Secure Module, see Executing Secure Module functions
0xE000001C	YES	YES	Used to send commands to Secure Module, see Executing Secure Module functions
0xE0000054	?	YES	SMC 0x12d, 0x135, 0x13B, Interrupt 0xC8
0xE0000058	?	YES	SMC 0x12d, 0x135, 0x13B, Interrupt 0xC8
0xE000005C	?	YES	SMC 0x12d, 0x135, 0x13B, Interrupt 0xC8
0xE0010000	YES	YES	Used by Cmep Command 0 to prepare Cmep to receive an address that contains the secure_kernel.enp.
0xE0010004	YES	?	Used by Cmep Command 0 to check if the value written in 0xE0010000 is okay (it should return a value <= 0).

Protocol

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

Bit End	Bit Start	Name	Description
31	16	Size	Size of the command buffer
18	13	-	Always 0
12	8	ID	Command ID
7	1	-	Always 0
0	0	Valid	Set to 1

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 0xE0000000 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 0xE0010000 and after that 0x0 to 0xE0010000, next, it will wait 0xE0010000 return 0 and 0xE0010004 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 0xE0000010. 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 0xE0000010 and then command 0x0 is written.

Command 1

This command is called by a subroutine of SceSblSmsched#sceSblSmSchedStopForTZS 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.

Offset	Size	Description
0x0	0x4	num_paddrs
0x4	0x4	paddr_list for Secure Module ELF
0x8	0x4	buf_0x40: some 0x40 buffer
0xC	0x4	ctx_0x4
0x10	0x4	ctx_0x8
0x14	0x4	ctx_0xC
0x1C	0x4	field_60: system version (2)
0x20	0x4	partition ID, ?Media Type?

Command 3

Load the previously suspended Secure Module.

Offset	Size	Description
0x0	0x4	num_paddrs
0x4	0x4	paddr_list for suspend buffer
0x8	0x4	buf_0x40: some 0x40 buffer
0xC	0x4	delayed_cmd

Command 4

Suspend the current Secure Module.

Offset	Size	Description
0x0	0x4	num_paddrs
0x4	0x4	paddr_list for suspend buffer
0x8	0x4	buf_0x40: some 0x40 buffer
0xC	0x4	delayed_cmd

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.

Offset	Size	Description
0x0	0x4	PA
0x4	0x4	length

Command 10

Set the encrypted signed revoke list.

Offset	Size	Description
0x0	0x4	num_paddrs
0x4	0x4	paddr_list for signed revoke list

Interrupts

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

• cry2arm0: handles interrupt 200. This is notifications about current sm status changes (loaded, unloaded, suspended, etc)
• cry2arm123: handles interrupts 201, 202, 203. This interrupt is sent once an sm command 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 SM is loaded, you can execute a functions it provides by writing into registers 0xE0000014, 0xE0000018, 0xE000001C paddr to 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, 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 0x00800000 to 0x00820000. Secure Modules are typically loaded to 0x0080B000. secure_kernel is loaded to 0x00800000.

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 Module scheduler running on ARM allows to have multiple tasks at once. The following provides an explanation of various states the task can be in.

ID	Description
1	Created / Suspended (Unloaded)
2	Running
3	Errored (Unloaded)
4	? (Unloaded)
5	? (Unloaded)
6	Start/resume requested
7	Suspending
8	?
9	?
10	?
11	Suspend requested
12	?

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