trezor-firmware/docs/bootloader.md

TREZOR Core Bootloader

TREZOR initialization is split into two stages. See Memory Layout for info about in which sectors each stage is stored.

First stage (boardloader) is stored in write-protected area, which means it is non-upgradable. Only second stage (bootloader) update is allowed.

First Stage - Boardloader

First stage checks the integrity and signatures of the second stage and runs it if everything is OK.

If first stage boardloader finds a valid second stage bootloader image on the SD card (in raw format, no filesystem), it will replace the internal second stage, allowing a second stage update via SD card.

The boardloader is special in that it is the device's write protected embedded code. The primary purpose for write protecting the boardloader is to make it the immutable portion that can defend against code-based attacks (e.g.- BadUSB) and bugs that would reprogram any/all of the embedded code. It assures that only verified signed embedded code is run on the device (and that the intended code is run, and not skipped). The write protection also provides some defense against attacks where the attacker has physical control of the device.

The boardloader must include an update mechanism for later stage code because if it did not, then a corruption/erasure of later stage flash memory would leave the device unusable (only the boardloader could run and it would not pass execution to a later stage that fails signature validation).

Second Stage - Bootloader

Second stage checks the integrity and signatures of the firmware and runs it if everything is OK.

If second stage bootloader detects a pressed finger on the display or there is no firmware loaded in the device, it will start in a firmware update mode, allowing a firmware update via USB.

Common notes

• Hash function used for computing data digest for signatures is BLAKE2s.
• Signature system is Ed25519 (allows combining signatures by multiple keys into one).
• All multibyte integer values are little endian.
• There is a tool called binctl which checks validity of the bootloader/firmware images including their headers.

Bootloader Format

TREZOR Core (second stage) bootloader consists of 2 parts:

1. bootloader header
2. bootloader code

Bootloader Header

Total length of bootloader header is always 512 bytes.

offset	length	name	description
0x0000	4	magic	firmware magic TRZB
0x0004	4	hdrlen	length of the bootloader header
0x0008	4	expiry	valid until timestamp (0=infinity)
0x000C	4	codelen	length of the bootloader code (without the header)
0x0010	1	vmajor	version (major)
0x0011	1	vminor	version (minor)
0x0012	1	vpatch	version (patch)
0x0013	1	vbuild	version (build)
0x0014	427	reserved	not used yet (zeroed)
0x01BF	1	sigmask	SatoshiLabs signature indexes (bitmap)
0x01C0	64	sig	SatoshiLabs aggregated signature

Firmware Format

TREZOR Core firmware consists of 3 parts:

1. vendor header
2. firmware header
3. firmware code

Vendor Header

Total length of vendor header is 84 + 32 * (number of pubkeys) + (length of vendor string rounded up to multiple of 4) + (length of vendor image) bytes rounded up to the closest multiple of 512 bytes.

offset	length	name	description
0x0000	4	magic	firmware magic TRZV
0x0004	4	hdrlen	length of the vendor header (multiple of 512)
0x0008	4	expiry	valid until timestamp (0=infinity)
0x000C	1	vmajor	version (major)
0x000D	1	vminor	version (minor)
0x000E	1	vsig_m	number of signatures needed to run the firmware from this vendor
0x000F	1	vsig_n	number of different pubkeys vendor provides for signing
0x0010	1	vtrust	level of vendor trust (0-100)
0x0011	15	reserved	not used yet (zeroed)
0x0020	32	vpub1	vendor pubkey 1
...	...	...	...
?	32	vpubn	vendor pubkey n
?	1	vstr_len	vendor string length
?	?	vstr	vendor string
?	?	vstrpad	padding to a multiple of 4 bytes
?	?	vimg	vendor image (in TOIf format)
?	?	reserved	padding to an address that is -65 modulo 512 (zeroed)
?	1	sigmask	SatoshiLabs signature indexes (bitmap)
?	64	sig	SatoshiLabs aggregated signature

Firmware Header

Total length of firmware header is always 512 bytes.

offset	length	name	description
0x0000	4	magic	firmware magic TRZF
0x0004	4	hdrlen	length of the firmware header
0x0008	4	expiry	valid until timestamp (0=infinity)
0x000C	4	codelen	length of the firmware code (without the header)
0x0010	1	vmajor	version (major)
0x0011	1	vminor	version (minor)
0x0012	1	vpatch	version (patch)
0x0013	1	vbuild	version (build)
0x0014	427	reserved	not used yet (zeroed)
0x01BF	1	sigmask	vendor signature indexes (bitmap)
0x01C0	64	sig	vendor aggregated signature

Various ideas

• Bootloader should be able to read vendor + firmware header and send info about FW to client in features message.
• Bootloader should not try to run firmware if there is not any.
• Storage wiping rule: Don't erase storage when old FW and new FW are signed using the same key set. Otherwise erase.
• Bootloader should send error to client when firmware update fails and allow client to try one more time. This prevents storage area erasure by accident.