tillitis-key/hw/application_fpga/fw/README.md

Firmware implementation notes

Introduction

This text is specific for the firmware, the piece of software in TKey ROM. For a more general description on how to implement device apps, see the TKey Developer Handbook.

Definitions

• Firmware: Software in ROM responsible for loading, measuring, and starting applications. The firmware is included as part of the FPGA bitstream and not replacable on a usual consumer TKey.
• Client: Software running on a computer or a mobile phone the TKey is inserted into.
• Device application or app: Software supplied by the client that runs on the TKey.

CPU modes and firmware

The TKey has two modes of software operation: firmware mode and application mode. The TKey always starts in firmware mode when it starts the firmware. When the application starts the hardware automatically switches to a more constrained environment: the application mode.

The TKey hardware cores are memory mapped but the memory access is different depending on mode. Firmware has complete access, except that the Unique Device Secret (UDS) words are readable only once even in firmware mode. The memory map is constrained when running in application mode, e.g. FW_RAM and UDS isn't readable, and several other hardware addresses are either not readable or not writable for the application.

When doing system calls from a device app the context switches back to firmware mode. However, the UDS is still not available, protected by two measures: 1) the UDS words can only be read out once and have already been read by firmware when measuring the app, and, 2) the UDS is protected by hardware after the execution leaves ROM for the first time.

See the table in the Developer Handbook for an overview about the memory access control.

Communication

The firmware communicates with the client using the UART_{RX,TX}_{STATUS,DATA} registers. On top of that is uses three protocols: The USB Mode protocol, the TKey framing protocol, and the firmware's own protocol.

To communicate between the CPU and the CH552 USB controller it uses an internal protocol, used only within the TKey, which we call the USB Mode Protocol. It is used in both directions.

Name	Size	Comment
Endpoint	1B	Origin or destination USB endpoint
Length	1B	Number of bytes following
Payload	See above	Actual data from or to firmware

The different endpoints:

Name	Value	Comment
CTRL	0x20	A USB HID special debug pipe. Useful for debug prints.
CDC	0x40	USB CDC-ACM, a serial port on the client.
HID	0x80	A USB HID security token device, useful for FIDO-type applications.

On top of the USB Mode Protocol is the TKey Framing Protocol which is described in the Developer Handbook.

The firmware uses a protocol on top of this framing layer which is used to bootstrap an application. All commands are initiated by the client. All commands receive a reply. See Firmware protocol in the Dev Handbook for specific details.

Memory constraints

Name	Size	FW mode	App mode
ROM	8 kByte	r-x	r
FW_RAM	4 kByte*	rw-	-
RAM	128 kByte	rwx	rwx

• FW_RAM is divided into the following areas:

• fw stack: 3824 bytes.
• resetinfo: 256 bytes.
• rest is available for .data and .bss.

Firmware behaviour

The purpose of the firmware is to load, measure, and start an application received from the client over the USB/UART.

The firmware binary is part of the FPGA bitstream as the initial values of the Block RAMs used to construct the FW_ROM. The FW_ROM start address is located at 0x0000_0000 in the CPU memory map, which is also the CPU reset vector.

Firmware state machine

This is the state diagram of the firmware. There are only four states. Change of state occur when we receive specific I/O or a fatal error occurs.

stateDiagram-v2
     S1: initial
     S2: loading
     S3: running
     SE: failed

     [*] --> S1

     S1 --> S1: Commands
     S1 --> S2: LOAD_APP
     S1 --> SE: Error

     S2 --> S2: LOAD_APP_DATA
     S2 --> S3: Last block received
     S2 --> SE: Error

     S3 --> [*]

States:

• initial - At start. Allows the commands NAME_VERSION, GET_UDI, LOAD_APP.
• loading - Expect application data. Allows only the command LOAD_APP_DATA.
• run - Computes CDI and starts the application. Allows no commands.
• fail - Stops waiting for commands, flashes LED forever. Allows no commands.

Commands in state initial:

command	next state
FW_CMD_NAME_VERSION	unchanged
FW_CMD_GET_UDI	unchanged
FW_CMD_LOAD_APP	loading

Commands in state loading:

command	next state
FW_CMD_LOAD_APP_DATA	unchanged or run on last chunk

See Firmware protocol in the Dev Handbook for the definition of the specific commands and their responses.

State changes from "initial" to "loading" when receiving LOAD_APP, which also sets the size of the number of data blocks to expect. After that we expect several LOAD_APP_DATA commands until the last block is received, when state is changed to "running".

In "running", the loaded device app is measured, the Compound Device Identifier (CDI) is computed, we do some cleanup of firmware data structures, enable the system calls, and finally start the app, which ends the firmware state machine. Hardware guarantees that we leave firmware mode automatically when the program counter leaves ROM.

The device app is now running in application mode. We can, however, return to firmware mode (excepting access to the UDS) by doing system calls. Note that ROM is still readable, but is now hardware protected from execution, except through the system call mechanism.

Golden path

Firmware loads the application at the start of RAM (0x4000_0000). It use a part of the special FW_RAM for its own stack.

When reset is released, the CPU starts executing the firmware. It begins in start.S by clearing all CPU registers, clears all FW_RAM, sets up a stack for itself there, and then jumps to main(). Also included in the assembly part of firmware is an interrupt handler for the system calls, but the handler is not yet enabled.

Beginning at main() it fills the entire RAM with pseudo random data and setting up the RAM address and data hardware scrambling with values from the True Random Number Generator (TRNG). It then waits for data coming in through the UART.

Typical expected use scenario:

1. The client sends the FW_CMD_LOAD_APP command with the size of the device app and the optional 32 byte hash of the user-supplied secret as arguments and gets a FW_RSP_LOAD_APP back. After using this it's not possible to restart the loading of an application.

2. If the the client receive a sucessful response, it will send multiple FW_CMD_LOAD_APP_DATA commands, together containing the full application.

3. On receivingFW_CMD_LOAD_APP_DATA commands the firmware places the data into 0x4000_0000 and upwards. The firmware replies with a FW_RSP_LOAD_APP_DATA response to the client for each received block except the last data block.

4. When the final block of the application image is received with a FW_CMD_LOAD_APP_DATA, the firmware measure the application by computing a BLAKE2s digest over the entire application. Then firmware send back the FW_RSP_LOAD_APP_DATA_READY response containing the digest.

5. The Compound Device Identifier (CDI) is then computed by doing a new BLAKE2s using the Unique Device Secret (UDS), the application digest, and any User Supplied Secret (USS) digest already received.

6. The start address of the device app, currently 0x4000_0000, is written to APP_ADDR and the size of the binary to APP_SIZE to let the device application know where it is loaded and how large it is, if it wants to relocate in RAM.

7. The firmware now clears the part of the special FW_RAM where it keeps it stack.

8. The interrupt handler for system calls is enabled.

9. Firmware starts the application by jumping to the contents of APP_ADDR. Hardware automatically switches from firmware mode to application mode. In this mode some memory access is restricted, e.g. some addresses are inaccessible (UDS), and some are switched from read/write to read-only (see the memory map).

If during this whole time any commands are received which are not allowed in the current state, or any errors occur, we enter the "failed" state and execute an illegal instruction. An illegal instruction traps the CPU and hardware blinks the status LED red until a power cycle. No further instructions are executed.

User-supplied Secret (USS)

USS is a 32 bytes long secret provided by the user. Typically a client program gets a secret from the user and then does a key derivation function of some sort, for instance a BLAKE2s, to get 32 bytes which it sends to the firmware to be part of the CDI computation.

Compound Device Identifier computation

The CDI is computed by:

CDI = blake2s(UDS, blake2s(app), USS)

In an ideal world, software would never be able to read UDS at all and we would have a BLAKE2s function in hardware that would be the only thing able to read the UDS. Unfortunately, we couldn't fit a BLAKE2s implementation in the FPGA at this time.

The firmware instead does the CDI computation using the special firmware-only FW_RAM which is invisible after switching to app mode. We keep the entire firmware stack in FW_RAM and clear the stack just before switching to app mode just in case.

We sleep for a random number of cycles before reading out the UDS, call blake2s_update() with it and then immediately call blake2s_update() again with the program digest, destroying the UDS stored in the internal context buffer. UDS should now not be in FW_RAM anymore. We can read UDS only once per power cycle so UDS should now not be available even to firmware.

Then we continue with the CDI computation by updating with an optional USS digest and finalizing the hash, storing the resulting digest in CDI.

Firmware system calls

The firmware provides a system call mechanism through the use of the PicoRV32 interrupt handler. They are triggered by writing to the trigger address: 0xe1000000. It's typically done with a function signature like this:

int syscall(uint32_t number, uint32_t arg1);

Arguments are system call number and upto 6 generic arguments passed to the system call handler. The caller should place the system call number in the a0 register and the arguments in registers a1 to a7 according to the RISC-V calling convention. The caller is responsible for saving and restoring registers.

The syscall handler returns execution on the next instruction after the store instruction to the trigger address. The return value from the syscall is now available in x10 (a0).

To add or change syscalls, see the syscall_handler() in syscall_handler.c.

Currently supported syscalls:

Name	Number	Argument	Description
RESET	1	Unused	Reset the TKey
SET_LED	10	Colour	Set the colour of the status LED
GET_VIDPID	12	Unused	Get Vendor and Product ID

Developing firmware

Standing in hw/application_fpga/ you can run make firmware.elf to build just the firmware. You don't need all the FPGA development tools. See the Developer Handbook for the tools you need. The easiest is probably to use your OCI image, ghcr.io/tillitis/tkey-builder.

Our version of qemu is also useful for debugging the firmware. You can attach GDB, use breakpoints, et cetera.

There is a special make target for QEMU: qemu_firmware.elf, which sets -DQEMU_CONSOLE, so you can use plain debug prints using the helper functions in lib.c like htif_puts() htif_putinthex() htif_hexdump() and friends. Note that these functions are only usable in qemu and that you might need to make clean before building, if you have already built before.

Test firmware

The test firmware is in testfw. It's currently a bit of a hack and just runs through expected behavior of the hardware cores, giving special focus to access control in firmware mode and application mode.

It outputs results on the UART. This means that you have to attach a terminal program to the serial port device, even if it's running in qemu. It waits for you to type a character before starting the tests.

It needs to be compiled with -Os instead of -O2 in CFLAGS in the ordinary application_fpga/Makefile to be able to fit in the 6 kByte ROM.