wiki: align PCR table to code, add note for TPM defend lock and further work needed - #246
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- PCR0: remove misleading SRTM parenthetical; coreboot SRTM uses PCR2 - PCR1: mark as not used - PCR2: add +data+blobs to reflect full measured boot scope - PCR3: document as runtime data slot (currently empty by design) - PCR4: add script citations (usb-init.sh:12, kexec-insert-key.sh:69, kexec-select-boot.sh:769) - PCR5: add script citation (insmod.sh:49-55) - PCR6: add script citation (qubes-measure-luks.sh:24) - PCR7: add script citations (cbfs-init.sh:55-56, uefi-init.sh:23) - PCR16: clarify as software-computed future PCR (not direct TPM extend) Code verification: initrd/bin/*.sh (all extend sites), config/coreboot-*.config (54 files), coreboot docs (PCRs 0-7 allocation table). Signed-off-by: Thierry Laurion <insurgo@riseup.net>
Signed-off-by: Thierry Laurion <insurgo@riseup.net>
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🟡 Changes recommended
A few documentation updates are still internally inconsistent/ambiguous (notably PCR7 wording vs later explanation and DA lockout phrasing), which can mislead readers.
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Pull request overview
Updates the Heads wiki documentation to better match observed TPM PCR extend behavior and related unseal failure modes, based on runtime measurements and referenced extend sites.
Changes:
- Updates the PCR 0–7 descriptions (including adding script references for extend sites) to reflect current measured-boot behavior.
- Adds a Dictionary Attack (DA) lockout note under “TPM_Unseal errors”.
- Ignores a local
.code-graph/directory in.gitignore.
File summaries
| File | Description |
|---|---|
| About/Keys.md | Revises PCR role descriptions and adds DA lockout documentation to align Keys page guidance with observed behavior. |
| .gitignore | Adds .code-graph/ to ignored paths for local tooling artifacts. |
Review details
- Files reviewed: 1/2 changed files
- Comments generated: 3
- Review effort level: Lite
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| 2: [coreboot measured boot](https://doc.coreboot.org/security/vboot/measured_boot.html#platform-configuration-registers) | ||
| — bootblock, romstage, ramstage, payload, bootsplash.jpg, fallback/*. | ||
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| 3: Nothing for the moment | ||
| 3: Runtime data slot — empty on all Heads builds. Coreboot can measure MRC | ||
| cache and hwinfo.hex here (DIMM-swap detection), currently disabled. |
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| 7: Heads CBFS files and UEFI binaries. Extended by `cbfs-init.sh`, | ||
| `uefi-init.sh`. |
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| Unseal can also fail due to TPM Dictionary Attack Lockout after too many failed | ||
| authentication attempts. On TPM 2, Heads enforces a 10-try limit with 1-hour | ||
| recovery during TPM reset. On TPM 1, DA behavior is vendor-specific — Heads | ||
| has no tooling to configure it. Pending: | ||
| [PR #2124](https://github.com/linuxboot/heads/pull/2124) adds user-facing DA | ||
| lockout detection and guidance. |
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Extends wiki iteration-1 (linuxboot/heads-wiki#246) to doc/tpm.md. - PCR 16: change from 'scratch use only' to 'unused' — calcfuturepcr is pure software, not a TPM extend to PCR 16 - kexec-seal-key.sh:274: fix stale comment that referenced PCR 16 — the code calls qubes-measure-luks.sh which extends PCR 6, not PCR 16 Signed-off-by: Thierry Laurion <insurgo@riseup.net>
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Extends wiki iteration-1 (linuxboot/heads-wiki#246) to doc/tpm.md. - PCR 16: change from 'scratch use only' to accurate history — was scratch PCR on TPM1 (PCR 23 on TPM2) in old calcfuturepcr; now unused after rewrite to replay from cbmem -L event log - kexec-seal-key.sh:274: fix stale comment that referenced PCR 16 — the code calls qubes-measure-luks.sh which extends PCR 6, not PCR 16 Signed-off-by: Thierry Laurion <insurgo@riseup.net>
This was referenced Sep 7, 2026
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Extends wiki iteration-1 (linuxboot/heads-wiki#246) to doc/tpm.md. - PCR 16: change from 'scratch use only' to accurate history — was scratch PCR on TPM1 (PCR 23 on TPM2) in old calcfuturepcr; now unused after rewrite to replay from cbmem -L event log - kexec-seal-key.sh:274: fix stale comment that referenced PCR 16 — the code calls qubes-measure-luks.sh which extends PCR 6, not PCR 16 Signed-off-by: Thierry Laurion <insurgo@riseup.net>
tlaurion
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Sep 7, 2026
Extends wiki iteration-1 (linuxboot/heads-wiki#246) to doc/tpm.md. - PCR 16: change from 'scratch use only' to accurate history — was scratch PCR on TPM1 (PCR 23 on TPM2) in old calcfuturepcr; now unused after rewrite to replay from cbmem -L event log - kexec-seal-key.sh:274: fix stale comment that referenced PCR 16 — the code calls qubes-measure-luks.sh which extends PCR 6, not PCR 16 Signed-off-by: Thierry Laurion <insurgo@riseup.net>
tlaurion
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Sep 8, 2026
Extends wiki iteration-1 (linuxboot/heads-wiki#246) to doc/tpm.md. - PCR 16: change from 'scratch use only' to accurate history — was scratch PCR on TPM1 (PCR 23 on TPM2) in old calcfuturepcr; now unused after rewrite to replay from cbmem -L event log - kexec-seal-key.sh:274: fix stale comment that referenced PCR 16 — the code calls qubes-measure-luks.sh which extends PCR 6, not PCR 16 Signed-off-by: Thierry Laurion <insurgo@riseup.net>
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Sep 8, 2026
Extends wiki iteration-1 (linuxboot/heads-wiki#246) to doc/tpm.md. - PCR 16: removed from security-model.md diagram — unused, not relevant to the security model - kexec-seal-key.sh:274: fix stale comment that referenced PCR 16 — the code calls qubes-measure-luks.sh which extends PCR 6, not PCR 16 Signed-off-by: Thierry Laurion <insurgo@riseup.net>
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Summary
Aligns the TPM PCR table in
About/Keys.mdwith actual runtime behavior verified againstcbmem -Loutput from a real Heads build.Fixes #245
Changes
PCR table corrections:
CONFIG_PCR_HWID,CONFIG_PCR_BOOT_MODE) but runtime shows zerousb-init.sh,kexec-insert-key.sh,kexec-select-boot.sh,sbin/insmod.sh,qubes-measure-luks.sh,cbfs-init.sh,uefi-init.sh)New content:
Evidence
All claims verified against:
cbmem -Loutput (real hardware TPM log)initrd/bin/*.sh(extend sites)config/coreboot-*.config(54 files)build/x86/coreboot-25.12/src/security/tpm/tspi/crtm.c(CBFS measurement routing)