On Mon, Feb 14, 2022 at 11:14:53AM -0800, Dhananjay Phadke wrote:
On 2/13/2022 5:13 PM, Andrew Jeffery wrote:
We can decouple HW RoT and runtime control on enforcing secure boot (requiring one or keys) on FIT image. Conflating two raises lot of questions.
I won't claim to be a security expert but I don't understand this statement. What are the "lots of questions" that are raised?
There's not much case for disabling HW RoT, which implies the bootloader (U-Boot or more) has to be trusted after board is manufactured (OTPstraps, especially OTPCFG0[6], are programmed).
There's indeed a case for disabling secure boot on OS FIT image -
Why wouldn't you want to replace the bootloader just as easily as you can replace the kernel / OS itself? I don't understand why this is more special than any other software. Bootloaders are replaced on "real" systems all the time. There are multiple efforts to be able to replace BIOS/UEFI with a free implementation as well.
I would consider the "HW RoT" to be the software in ROMs and not anything which can be replaced, like u-boot. Why are you extending it to include u-boot?
If bootloader is trusted, it's possible to remotely push the policy to disable runtime FIT image secure boot. Such policy push must use secure transport (someway authenticated) and storage (simplest U-Boot env). This is helpful in cases like booting diagnostic images if board has to be RMA'ed and diagnostic images won't be signed by production keys.
For second-hand / recycled hardware, I'm not sure the bootloader _is_ trusted. It is also possible that I punt on some aspects of supply-chain security and simply replace it all when it arrives in my hands. ie. If I can securely replace all the bits, I really don't care if it was tampered with in transit.
There's a key-requirement policy already implemented [1].
[1] https://lore.kernel.org/u-boot/cover.1597643014.git.thiruan@linux.microsoft....
Board code can patch "required-policy" = none at runtime based appropriate logic.
Regards, Dhananjay
With that in mind:
To escape the manufacturer's key-chain for owner-controlled signatures the concept is the manufacturer-signed SPL (or u-boot payload) will load keys from an external, write-protected EEPROM. These keys are used to verify the next element of the boot process, providing user control.
To configure owner-controlled keys the EEPROM write-protect must be disabled. This may, for example, be done via a physical jumper. If left with write-protection disabled the matching public key for the signature on the payload can arbitrarily be installed into the EEPROM which makes secure-boot verification moot. The patch avoids the run-around in this last behaviour by providing a platform hook to read the state of what is effectively the EEPROM write-protect pin.
Isn't this jumper proposal just like the TCG Physical Presence requirements? This is a software implementation and requires a particular hardware design for it to be done right, but it seems to be along the same lines.