We have a bootloader. It does the usual code signature checking, but also offers some security features used during runtime. Part of this is keeping some bytes secret in the long term. It can never be field upgraded, and yet plays an important part in that process.
This code is linked separately from other executables, and resides in its own reserved area at the start of flash memory. That area is protected from readback using chip features: "Proprietary Code Read-Out Protection (PCROP)" aka. firewall.
A very limited amount of security-sensitive code resides here. It protects your currency, but only indirectly. It's more about making your key storage per-system unique.
the most helpful file here is
bootloader.lsswhich is generated in build process
using OpenOCD is prefered for lower level code like this (not GDB)
stm32l4x.cpu arm disassemble 0x000008 10 thumbis very helpful
you can power cycle the board (to enter/exit DFU) and OpenOCD keeps working
for consistent reading of state, do this:
- power cycle
wipe chip with:
stm32l4x mass_erase 0in openocd monitor to bulk-erase whole chip
To clear flash with write protect on... FLASH regs at 0x40022000 base FLASH->CR = 0x40022014 FLASH->WRP1AR = 0x4002202c
have DFU active. doesn't work from running
expect 0x40000000, if it's 0xc0000000, can't work; reboot w/ DFU pressed, to fix
ignore warning about "power cycle" from this:
stm32l4x unlock 0
expect 0 from this:
disable all write-protect (bank 1, A region)
mww 0x4002202c 0xff00ffff mww 0x40022030 0xff00ffff
mww 0x40022014 0x20000
read back in OB (expect ff00ffff NOT ff0fff00)
launch changes? (causes weird reset)
mww 0x40022014 0x8000000
"stm32l4x.cpu mdb" is nice hexdump, much better than regular mdb
- https://github.com/B-Con/crypto-algorithms for sha256 code.
Reset Vector + Callgate Hacking
using OpenOCD monitor talking to real chip
using OpenOCD disassembler
working in RAM @ 0x10000000
need to encode the reset vector (into boot loader) and also be a valid sequence of thumb insts
best if it doesn't mangle R1-R4,LR so they can be used for callgate API
I require: 08000 xxxx where - LSB of x is one (thumb mode), - and (x) is less than 0x7800 - preferably x is small
mwh 0x10000000 0x00b1 mwh 0x10000002 0x0800 stm32l4x.cpu arm disassemble 0x10000000 2 thumb
0x10000000 0x00b1 LSLS r1, r6, #0x02 0x10000002 0x0800 LSRS r0, r0, #0x20
"LSRS r0, r0, #0x20" will be unavoidable
here's a nice sequence: 0x10000000 0x007f LSLS r7, r7, #0x01 0x10000002 0x0800 LSRS r0, r0, #0x20
however, it's a no-go because once the firewall is activated, the reset vector can't work: it doesn't do the callgate right. It may be fetching the reset vector and/or the stack ptr before dying but it's hard to tell. Also tried jumping to the callgate (0x05) as the reset vector but that didn't work either.
lesson learned: we must keep the bootloader readonly and block flash writes because other wise the DFU can/will (partially) erase the 0x100 or so non-firewalled bytes. Of course the datasheet says that too, and states level 2 write protection is needed.
- measure OLED reset and CS pulse lengths, and SPI clk during boot w/ internal RC oscilator
- HAL code for SPI should be removed and replaced with a few one-liners
- GPIO code maybe removed as well?
- for 0123783355b2ab9dee: