b0c5c6aad8
We'd like to measure various additional things into PCRs, but all available ones to the OS are already used for various purposes. Hence, let's introduce a new concept of "NV Index based PCRs", i.e. let's use TPM2 nv indexes of type TPM2_NT_EXTEND that mostly behave like real PCRs, but which we can allocate relatively freely from the nv index space. Let's call these "fake" PCRs "NvPCRs". My original intention was to get a fixed NV index range assigned from the TCG, either for Linux or for systemd as a project, but this stalled with no further updates from the TCG for more than a year and a half now. I was told an NV index range to use though, even if it never was officially assigned, hence this PR uses this by default. But the range is configurable at build time, on purpose, so that downstreams have some flexibility to change this if they want. To abstract the actual nvindex number away we introduce a naming concept, so that nvindexes are referenced by name string rather than number. NvPCRs are defined in little JSON snippets in /usr/lib/nvpcr/*.nvpcr, that match up index number and name, as well as pick a hash algorithm. There's one complication: these nvindex (like any nvindex) can be deleted by anyone with access to the TPM, and then be recreated. This could be used to reset the NvPCRs to zero during runtime, which defeats the whole point of them. Our way out: we measure a secret as first thing after creation into the NvPCRs. (Or actually, we measure a per-NvPCR secret we derive from a system secret via an HMAC of the NvPCR name) and the nvindex handle). This "anchoring" secret is stored in /run/ + /var/lib/ + ESP/XBOOTLDR (the latter encrypted as credential, locked to the TPM), to make it available at the whole runtime of the OS.
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