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docs: explain the role of PCR[10] and why it is not reproducible (#2011)

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Malte Poll 2023-07-04 16:41:01 +02:00 committed by GitHub
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docs/docs/architecture/attestation.md
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@ -144,7 +144,7 @@ The latter means that the value can be generated offline and compared to the one
| 7 | Secure Boot State | Azure, Constellation Bootloader | No |
| 8 | - | - | - |
| 9 | initramfs | Linux Kernel | Yes |
| 10 | Reserved | - | No |
| 10 | Userspace[^1] | Linux IMA[^1] | No[^1] |
| 11 | Reserved for Unified Kernel Image components | (Constellation Bootloader) | Yes |
| 12 | Kernel command line | Constellation Bootloader | Yes |
| 13 | Reserved | (Constellation Bootloader) | Yes |
@ -177,7 +177,7 @@ The latter means that the value can be generated offline and compared to the one
| 7 | GCP Secure Boot Policy | GCP, Constellation Bootloader | No |
| 8 | - | - | - |
| 9 | initramfs | Linux Kernel | Yes |
| 10 | Reserved | - | No |
| 10 | Userspace[^1] | Linux IMA[^1] | No[^1] |
| 11 | Reserved for Unified Kernel Image components | (Constellation Bootloader) | Yes |
| 12 | Kernel command line | Constellation Bootloader | Yes |
| 13 | Reserved | (Constellation Bootloader) | Yes |
@ -209,7 +209,7 @@ The latter means that the value can be generated offline and compared to the one
| 7 | Secure Boot Policy | AWS, Constellation Bootloader | No |
| 8 | - | - | - |
| 9 | initramfs | Linux Kernel | Yes |
| 10 | Reserved | - | No |
| 10 | User space | Linux IMA | No[^1] |
| 11 | Reserved for Unified Kernel Image components | (Constellation Bootloader) | Yes |
| 12 | Kernel command line | Constellation Bootloader | Yes |
| 13 | Reserved | (Constellation Bootloader) | Yes |
@ -308,3 +308,9 @@ flowchart LR
D["Public key"]-- "verifies" -->E["Runtime measurements"]
E["Runtime measurements"]-- "verify" -->F["Constellation cluster"]
```
## References
[^1]: Linux IMA produces runtime measurements of user space binaries.
However, these measurements aren't deterministic and thus, PCR\[10] can't be compared to a constant value.
Instead, a policy engine must be used to verify the TPM event log against a policy.