[PATCH] image: Control FIT signature verification at runtime

[Patrick Williams]

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.com/
> 
> 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.

-- 
Patrick Williams
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