efi/preinstall: Fall back to checking the BootGuard status MSR in HAP mode#535
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chrisccoulson merged 2 commits intocanonical:masterfrom Apr 9, 2026
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… mode Access to the HFSTS registers via the HECI is not possible on systems that use Intel's High Assurance Platform mode. This means that we can't check the BootGuard policy. However, the startup ACM mirrors some BootGuard policy settings to a MSR, so we can check this as a fallback in this case. There are some limitations here - it's not possible to check some bits that indicate the platform has properly transitioned out of manufacturing mode, and the error enforcement bits of the BootGuard policy are not available either. However, there are no combinations of the other policy bits that are duplicated with different error enforcement bits for any of the recognized profiles, so this doesn't matter. Note that the reporter of this issue also noted that turning on HAP mode resulted in an unexpected EV_EFI_ACTION event being measured to PCR7 on their system. This doesn't fix that yet because I'm still waiting on a response from the reporter.
| // -*- Mode: Go; indent-tabs-mode: t -*- | ||
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| /* | ||
| * Copyright (C) 2024 Canonical Ltd |
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I'll change that :)
| // where these bits are reused with different error enforcement policies. | ||
| f := s&bootGuardFACB > 0 | ||
| v := s&bootGuardVerifiedBoot > 0 | ||
| m := s&bootGuardMeasuredBoot > 0 |
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I suppose this measured boot has nothing to do with TPM, or does it?
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It implies that the ACM is the root of trust for measurement rather than the EFI firmware measuring itself.
valentindavid
approved these changes
Apr 9, 2026
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Access to the HFSTS registers via the HECI is not possible on systems
that use Intel's High Assurance Platform mode. This means that we can't
check the BootGuard policy. However, the startup ACM mirrors some
BootGuard policy settings to a MSR, so we can check this as a fallback
in this case.
There are some limitations here - it's not possible to check some bits
that indicate the platform has properly transitioned out of
manufacturing mode, and the error enforcement bits of the BootGuard
policy are not available either. However, there are no combinations of
the other policy bits that are duplicated with different error
enforcement bits for any of the recognized profiles, so this doesn't
matter.
Note that the reporter of this issue also noted that turning on HAP mode
resulted in an unexpected EV_EFI_ACTION event being measured to PCR7 on
their system. This doesn't fix that yet because I'm still waiting on a
response from the reporter.