tpm: pair the PCR bank algorithm with its digest size

[dgarske]

Found while adding SHA2-384 measured boot for a Tiger Lake TPM target.

WOLFBOOT_TPM_PCR_ALG and WOLFBOOT_TPM_PCR_DIG_SZ were defined together under a single #ifndef on the algorithm. Overriding only the algorithm therefore left the digest size undefined, and an undefined macro is 0 to the preprocessor, so the downstream digest-size checks silently changed meaning rather than failing to build.

This selects the pair with one switch - WOLFBOOT_TPM_PCR_SHA384 chooses SHA2-384 (digest size 48), otherwise SHA2-256 (digest size 32) - and adds an #error when exactly one of the two macros is defined, so a half-specified override is caught at compile time. The algorithm itself cannot be validated with #if: TPM_ALG_SHA256 and friends are enum constants, not macros, so any #if comparison against them evaluates 0 == 0 and is always true.

Note: a SHA2-384 PCR bank exists on parts like the Infineon SLB9672 but not the SLB9670; selecting it on a part without that bank makes every extend fail, which measured boot treats as fatal.

[Copilot]

Copilot review overview

🟡 Changes recommended

SHA-384 PCR selection can cause existing 32-byte measurement buffers to be read as 48-byte inputs.

Get a fresh assessment by requesting another Copilot review.

Review effort: Lite
Findings: 1

Open (1)

• Prevent TPM PCR digest-size mismatch and hash buffer overread · New

What changed in this PR

This PR makes TPM PCR algorithm and digest-size configuration explicit and adds SHA-384 PCR-bank selection.

Changes:

• Detects half-specified PCR overrides at compile time.
• Adds WOLFBOOT_TPM_PCR_SHA384 for SHA-384/48-byte PCRs.
• Retains SHA-256/32-byte defaults.

File	Description
include/​tpm.h	Defines PCR algorithm and digest-size selection.

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.