On Tue, 15 Feb 2022, at 05:44, Dhananjay Phadke wrote:
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:
- The typical approach - No owner control: Manufacturer supplied
firmware with secure-boot always enabled
- 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-12819c8c40ef@linux.m...
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:
- All secure-boot configuration is held in OTP memory integrated into
the SoC
- The OTP memory must be write-protected to prevent attackers
installing arbitrary keys without physical presence
- The OTP is write-protected by configuration held in the OTP.
The consequence with respect to 2. is that the system manufacturer either must:
Program and write-protect the OTP during production, or
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:
- The manufacturer signs arbitrary SPLs upon request. This requires
trust from both parties and potentially a lot of auditing focus from the manufacturer.
- 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:
- Software security
- 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.
Right. They are decoupled. What I'm proposing in the patch only affects FIT verification.
Cheers,
Andrew