Hi,
I'm trying to add a way to lock the SPI flash to be read-only via software *after* coreboot boots. The scenario is basically with using Heads, you could authenticate to it (with a yubikey/nitrokey/librem key) then be able to flash a new rom (update your BIOS), but once you boot an OS, Heads would first lock the flash so it can't be written to. This should add some security to avoid any malware writing to the flash, or someone booting into a USB stick and using that to flash a malicious BIOS, but still gives the user the freedom of updating their flash whenever they want to.
The problem is that I can't make the flash read-only because the SPI interface is already locked down by coreboot (see src/soc/intel/skylake/lockdown.c and src/soc/intel/common/block/fast_spi/fast_spi.c).
There's a couple of things happening here : First, the FLOCKDN bit is set which prevents us from enabling the write protection. the BIOS Interface lock down is controlled by the chipset_lockdown config variable, but the FLOCKDN is not behind a config variable. The second thing is that if I wanted to use the protected ranges feature to lock specific regions, they are all getting locked using the discrete lock register even while being unused. The locking of the protected ranges was added in this change : https://review.coreboot.org/c/coreboot/+/21064 and it passed without notice among the move that the commit was supposed to do.
The commit states that the lockdown is meant to "support platform security guidelines", but I think that this is not actually good because coreboot leaves everything read-write and locks down the registers so we can't make it read-only. I think that the security guidelines would say to disable the write protection before locking the registers down.
Either way, I'm going to need to add a way to make this configurable, so my main questions here are : - Should I create a new config variable to decide on whether or not to lock the spi registers and another one to decide on whether or not to lock the protected ranges ? - Should I make the chipset_lockdown (currently used for locking the BIOS CNTL register from LPC and SPI controllers) into an OR-ed flags variable where we can say : chipset_lockdown = LOCKDOWN_COREBOOT | LOCKDOWN_SPI | LOCKDOWN_PROTECTED_RANGES ? - Should I make a single new config variable to decide what to lockdown (LPC_BIOS, SPI_BIOS, SPI_BAR, SPI_PROTECTED_RANGES) and only set them if the CHIPSET_LOCKDOWN_COREBOOT is set ? And if chipset_lockdown is set to CHIPSET_LOCKDOWN_FSP not lockdown anything at all ? - Do we want to keep the protected ranges locked down at all, have them configurable or completely remove that as I don't see the point in using the discrete lock register ?
Once I see a consensus on what's the best way to move forward, I'll implement it and push it for review.
Note: I think these only affect hardware sequencing though so I assume someone could always use software sequencing to do the writes. As long as the FLOCKDN bit isn't set though, I could remove all write-related opcodes from the software sequencing register, which would also prevent someone using swseq to do writes.
Thanks, Youness.