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doc: remove unsupported client PCR 0 measurement claim #2210

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tlaurion merged 1 commit into from
Sep 17, 2026
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doc: remove unsupported client PCR 0 measurement claim #2210

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7 changes: 3 additions & 4 deletions doc/tpm.md
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Expand Up @@ -173,10 +173,9 @@ and 3 are zero while their coreboot features stay disabled. PCR 0 is normally
zero and holds a measurement only when the platform is provisioned with a
BootGuard boot profile that includes measurement; in that case the BootGuard ACM
measures the Key Manifest, Boot Policy Manifest, IBB and policy data into PCR 0
at locality 3. Historically this came from the server TXT path, but BootGuard and
CBnT can measure on client silicon as well; most client machines ship verified
boot only, so PCR 0 stays zero. Any extension of these PCRs changes the value
recorded in the policy and breaks the seal.
at locality 3; a platform provisioned without such a profile leaves PCR 0
unchanged. Any extension of these PCRs changes the value recorded in the policy
and breaks the seal.

For the measurement behavior and its history see the [heads PR #1172
discussion](https://github.com/linuxboot/heads/pull/1172) and the [Dasharo PCR
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