GM45 Thinkpad Internal flashing research
The goal is to be able to flash the Lenovo GM45/GS45 Thinkpads like the Thinkpad X200 Internally(without the need of an external flasher), with flashrom, after having booted the default boot firmware.
The Lenovo X200 uses the following mechanisms to prevent internal reflashing:
- Flash descriptor: Set the flash descriptor read-only, locks the ME, and platform regions.
- PR registers: Sets the BIOS bootblock read-only and prevent access to the platform region
- The BUC.TS register is locked.
- If we remove the flash descriptor read-only protection we are able to easily reflash coreboot, but:
- The flash descriptor restrictions may be able to be lifted by using the GPIO33, but accessing that pin is very difficult and has huge probability of breaking the board.
- Finding a command to send to the ME to unlock it is very unlikely, as it is only supposed to work when the management engine is in manufacture-mode. The Me is not in manufacture-mode on production laptops.
- Find a way to disable or crash the ME would probably have no effect at all on flash protections
- Executing code in SMM would have no impact as it woun't permit to remove flash read-only protections.
When the computer boots, the bootblock sets the PR registers, which has the effect of making itself read-only. If there are conditions in which the bootblock doesn't set the PR registers, finding, understanding, and using them would allow us to reflash coreboot internally when booted from the default boot firmware.
There are several approaches that could help finding them:
- There are modded boot firmwares that aim to disable the WiFi card whitelist: While they only update some sections of the flash image, they also come with some unused bootblock and flash descriptor that respectively don't set the PR register and that disable the Management Engine:
- Comparing them with the original boot firmware might lead to interesting results.
- It might be possible to run the bootblock with SerialICE to find what it does.