On 2/13/2022 5:13 PM, Andrew Jeffery wrote:
Right, I think this question is an indication that I could write a more
informative commit message, so if we converge on something acceptable
I'll update it. Let me provide some more context:
As mentioned above this is motivated by use with BMCs, specifically on
the ASPEED AST2600, in some specific platform contexts.
Platforms can be designed with two styles of firmware management in
mind:
1. The typical approach - No owner control: Manufacturer supplied
firmware with secure-boot always enabled
2. The atypical approach - Full owner control: Owner-controlled firmware
with secure-boot optionally enabled
For quite a few contributing to OpenBMC, the manufacturer and the owner
are the same, and so the typical approach is a good match. It probably
is the use case Dhananjay was considering[1]. It also caters to the
traditionally closed-source firmware ecosystem where manufacturer
control is retained.
[1]https://lore.kernel.org/openbmc/016ae207-2ecb-1817-d10e-12819c8c4...@linux.microsoft.com/
The second approach requires open-source firmware stacks combined with
platforms designed to enable owner control. There are some ecosystems
that allow this (e.g. OpenPOWER). On the host side for those systems
it's possible to secure-boot them using firmware and kernels signed
entirely and only by user-controlled keys. We're looking to enable this
for the BMC side too, as much as we can.
For completeness (i.e. stating this to make the argument self-contained,
not implying that you're unaware of it), for secure-boot to be
meaningful at a given point in the boot process we need all previous
elements of the boot process to have been verified. That is, it's not
enough to verify u-boot if the u-boot SPL is not verified.
Where such systems use the AST2600, limitations of the AST2600
secure-boot design come into play:
3. All secure-boot configuration is held in OTP memory integrated into
the SoC
4. The OTP memory must be write-protected to prevent attackers
installing arbitrary keys without physical presence
5. The OTP is write-protected by configuration held in the OTP.
The consequence with respect to 2. is that the system manufacturer
either must:
6. Program and write-protect the OTP during production, or
7. Ship the system with the OTP in a vulnerable state.
We figure the latter isn't desirable which means dealing with
limitations of the former.
If the OTP is programmed (with the required public keys) and
write-protected by the manufacturer, then when configured as a
secure-boot root-of-trust, the AST2600 must only boot an SPL image
signed by the manufacturer. From here there are two options for owner
control:
8. The manufacturer signs arbitrary SPLs upon request. This requires
trust from both parties and potentially a lot of auditing focus from the
manufacturer.
9. The manufacturer publishes the source for the signed u-boot SPL
binary in a fashion that allows reproducible builds for verification by
inspection. Firmware signed by owner-controlled keys can only be applied
beyond this boot stage.
Despite the compromise, the latter approach seems to be the most
reasonable in the circumstances.
Again for completeness, broadly, security can be divided into two
categories:
10. Software security
11. Physical security
Controlling secure-boot in a way that requires physical presence rules
out the ability to influence the boot process via (remote) software
attacks. This is beneficial. The approach at the platform level does not
yet solve for physical security, however given this is motivated by use
on BMCs, physical security concerns could be viewed as taken care of in
the sense that the systems are likely installed in a datacenter or some
other controlled environment.
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.
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 -
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.
There's a key-requirement policy already implemented [1].
[1]
https://lore.kernel.org/u-boot/cover.1597643014.git.thir...@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.
