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two pcr spec fixes #221

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Merged
poettering merged 2 commits into from
Jul 2, 2026
Merged

two pcr spec fixes #221

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7215060
specs: tweak wording around OS separator
poettering Jun 30, 2026
a5dbe8d
pcr-registry: document systemd-stub's SMBIOS measurement
poettering Jun 30, 2026
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47 changes: 33 additions & 14 deletions specs/linux_tpm_pcr_registry.md
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Original file line number Diff line number Diff line change
Expand Up @@ -94,7 +94,7 @@ In both cases it is important that data measured into the PCRs is carefully chos
<td class=number></td>
<td class=systemd><code>systemd-pcrosseparator.service 🚀</code></td>
<td>Userspace</td>
<td><code>os-separator</code> barrier when leaving initrd</td>
<td><code>os-separator</code> barrier when the TPM device becomes available to userspace</td>
<td><code>/run/log/systemd/tpm2-measure.log</code></td>
<td>n/a</td>
</tr>
Expand All @@ -108,11 +108,20 @@ In both cases it is important that data measured into the PCRs is carefully chos
<td>n/a</td>
</tr>

<tr>
<td class=number></td>
<td class=systemd><code>systemd-stub 🚀</code></td>
<td>UEFI Stub</td>
<td>Measurement of SMBIOS Type 1, 2, 11 as set when the UEFI stub takes over</td>
<td>UEFI TPM event log</td>
<td>n/a</td>
</tr>

<tr>
<td class=number></td>
<td class=systemd><code>systemd-pcrosseparator.service 🚀</code></td>
<td>Userspace</td>
<td><code>os-separator</code> barrier when leaving initrd</td>
<td><code>os-separator</code> barrier when the TPM device becomes available to userspace</td>
<td><code>/run/log/systemd/tpm2-measure.log</code></td>
<td>n/a</td>
</tr>
Expand All @@ -130,7 +139,7 @@ In both cases it is important that data measured into the PCRs is carefully chos
<td class=number></td>
<td class=systemd><code>systemd-pcrosseparator.service 🚀</code></td>
<td>Userspace</td>
<td><code>os-separator</code> barrier when leaving initrd</td>
<td><code>os-separator</code> barrier when the TPM device becomes available to userspace</td>
<td><code>/run/log/systemd/tpm2-measure.log</code></td>
<td>n/a</td>
</tr>
Expand All @@ -148,7 +157,7 @@ In both cases it is important that data measured into the PCRs is carefully chos
<td class=number></td>
<td class=systemd><code>systemd-pcrosseparator.service 🚀</code></td>
<td>Userspace</td>
<td><code>os-separator</code> barrier when leaving initrd</td>
<td><code>os-separator</code> barrier when the TPM device becomes available to userspace</td>
<td><code>/run/log/systemd/tpm2-measure.log</code></td>
<td>n/a</td>
</tr>
Expand All @@ -166,7 +175,7 @@ In both cases it is important that data measured into the PCRs is carefully chos
<td class=number></td>
<td class=systemd><code>systemd-pcrosseparator.service 🚀</code></td>
<td>Userspace</td>
<td><code>os-separator</code> barrier when leaving initrd</td>
<td><code>os-separator</code> barrier when the TPM device becomes available to userspace</td>
<td><code>/run/log/systemd/tpm2-measure.log</code></td>
<td>n/a</td>
</tr>
Expand All @@ -193,7 +202,7 @@ In both cases it is important that data measured into the PCRs is carefully chos
<td class=number></td>
<td class=systemd><code>systemd-pcrosseparator.service 🚀</code></td>
<td>Userspace</td>
<td><code>os-separator</code> barrier when leaving initrd</td>
<td><code>os-separator</code> barrier when the TPM device becomes available to userspace</td>
<td><code>/run/log/systemd/tpm2-measure.log</code></td>
<td>n/a</td>
</tr>
Expand All @@ -202,7 +211,7 @@ In both cases it is important that data measured into the PCRs is carefully chos
<td class=number><p style="text-align: right"><strong>6</strong></p></td>
<td class=systemd><code>systemd-pcrosseparator.service 🚀</code></td>
<td>Userspace</td>
<td><code>os-separator</code> barrier when leaving initrd</td>
<td><code>os-separator</code> barrier when the TPM device becomes available to userspace</td>
<td><code>/run/log/systemd/tpm2-measure.log</code></td>
<td>n/a</td>
</tr>
Expand All @@ -220,7 +229,7 @@ In both cases it is important that data measured into the PCRs is carefully chos
<td class=number></td>
<td class=systemd><code>systemd-pcrosseparator.service 🚀</code></td>
<td>Userspace</td>
<td><code>os-separator</code> barrier when leaving initrd</td>
<td><code>os-separator</code> barrier when the TPM device becomes available to userspace</td>
<td><code>/run/log/systemd/tpm2-measure.log</code></td>
<td>n/a</td>
</tr>
Expand Down Expand Up @@ -274,7 +283,7 @@ In both cases it is important that data measured into the PCRs is carefully chos
<td class=number></td>
<td class=systemd><code>systemd-pcrosseparator.service 🚀</code></td>
<td>Userspace</td>
<td><code>os-separator</code> barrier when leaving initrd</td>
<td><code>os-separator</code> barrier when the TPM device becomes available to userspace</td>
<td><code>/run/log/systemd/tpm2-measure.log</code></td>
<td>n/a</td>
</tr>
Expand Down Expand Up @@ -319,7 +328,7 @@ In both cases it is important that data measured into the PCRs is carefully chos
<td class=number></td>
<td class=systemd><code>systemd-pcrosseparator.service 🚀</code></td>
<td>Userspace</td>
<td><code>os-separator</code> barrier when leaving initrd</td>
<td><code>os-separator</code> barrier when the TPM device becomes available to userspace</td>
<td><code>/run/log/systemd/tpm2-measure.log</code></td>
<td>n/a</td>
</tr>
Expand All @@ -337,7 +346,7 @@ In both cases it is important that data measured into the PCRs is carefully chos
<td class=number></td>
<td class=systemd><code>systemd-pcrosseparator.service 🚀</code></td>
<td>Userspace</td>
<td><code>os-separator</code> barrier when leaving initrd</td>
<td><code>os-separator</code> barrier when the TPM device becomes available to userspace</td>
<td><code>/run/log/systemd/tpm2-measure.log</code></td>
<td>n/a</td>
</tr>
Expand All @@ -355,7 +364,7 @@ In both cases it is important that data measured into the PCRs is carefully chos
<td class=number></td>
<td class=systemd><code>systemd-pcrosseparator.service 🚀</code></td>
<td>Userspace</td>
<td><code>os-separator</code> barrier when leaving initrd</td>
<td><code>os-separator</code> barrier when the TPM device becomes available to userspace</td>
<td><code>/run/log/systemd/tpm2-measure.log</code></td>
<td>n/a</td>
</tr>
Expand Down Expand Up @@ -390,6 +399,12 @@ In both cases it is important that data measured into the PCRs is carefully chos

PCR 0 changes on firmware updates; PCR 1 changes on basic hardware/CPU/RAM replacements.

[systemd-stub](https://www.freedesktop.org/software/systemd/man/latest/systemd-stub.html) measures SMBIOS
Type 1, 2, 11 objects when it is executed. These objects are used to parameterize the OS, and hence
important to measure. Some firmwares do not measure SMBIOS structures on their own, hence this is supposed to
fill the gap on such systems. Note that the Type 1 field for the wakeup source is masked to zero before the
measurement is made, in order to provide stable measurements independently of system wakeup sources.

PCR 4 changes on boot loader updates.
The shim project will measure the PE binary it chain loads into this PCR.
If the Linux kernel is invoked as UEFI PE binary, it is measured here, too.
Expand All @@ -402,9 +417,13 @@ PCR 5 changes when partitions are added, modified, or removed.

PCR 7 changes when UEFI SecureBoot mode is enabled/disabled, or firmware certificates (PK, KEK, db, dbx, …) are updated.
The shim project will measure most of its (non-MOK) certificates and SBAT data into this PCR.

[systemd-pcrosseparator.service](https://www.freedesktop.org/software/systemd/man/systemd-pcrosseparator.service.html)
extends PCRs 0–7, 9, 12–14 with the literal word `os-separator` as a barrier when leaving the initrd,
so the PCRs differ between the initrd and the main OS.
extends PCRs 0–7, 9, 12–14 with the literal word `os-separator` as a barrier measurement when userspace first
sees the TPM device, so that firmware and OS measurements are clearly isolated from each other and
distinguishable. This provides protection for systems where the firmware does not generate the UEFI-mandated
measurement on its own, or doesn't natively support TPMs at all (for example because the TPM is emulated in
software).

PCR 11 and 15 as shown in the list above are used by multiple components of systemd.
These are not conflicting uses;
Expand Down
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