[systemd-devel] Trying to understand change in PCR 4 extension behavior

[Kyle Rose]

I've been developing and using my own measured boot solution for a few
years now, based on the EFI stub provided by systemd. Sometime between
v247 and v252, the behavior changed in a way that I have not been able
to track down to a specific change, partly because I haven't been able
to pinpoint where the new measurement has been implemented.

In v247, the boot log ends by extending PCR 4 with a single very
simple BSA event:

- EventNum: 25
  PCRIndex: 4
  EventType: EV_EFI_BOOT_SERVICES_APPLICATION
  DigestCount: 2
  Digests:
  - AlgorithmId: sha1
    Digest: "9b87079d73e8e2135b43fb41545c5a1917672a50"
  - AlgorithmId: sha256
    Digest: "61f58b66936e962f1872c2b2618c4578d905a60cabcadfe1e707a7bb1bbf862c"
  EventSize: 168
  Event:
    ImageLocationInMemory: 0x82c78018
    ImageLengthInMemory: 73463647
    ImageLinkTimeAddress: 0x0
    LengthOfDevicePath: 136
    DevicePath:
'02010c00d041030a0000000001010600001d01010600000003171000010000000025385191b2d71d04012a00010000000008000000000000002811000000000025707467aecfea11850ecdf2cff0384c0202040432005c004500460049005c00440045004200490041004e005c0045004d0042004f004f0054002e0045004600490000007fff0400'

However, in v252, the corresponding event occurs earlier in the log
and (after some measurements extending PCR 11) is followed by another
BSA event extending PCR 4 with a DevicePath I can't parse from a call
I can't seem to find in the systemd source code:

- EventNum: 34
  PCRIndex: 4
  EventType: EV_EFI_BOOT_SERVICES_APPLICATION
  DigestCount: 2
  Digests:
  - AlgorithmId: sha1
    Digest: "9a3c68bb105e4c4e70cbc3375bd45d616e220586"
  - AlgorithmId: sha256
    Digest: "36e49f2a0c246db5836b85319e7b2ae04690aca40227895902870a54a054c78b"
  EventSize: 56
  Event:
    ImageLocationInMemory: 0xb7c36000
    ImageLengthInMemory: 7793888
    ImageLinkTimeAddress: 0x1000000
    LengthOfDevicePath: 24
    DevicePath: '04031400f8d1c555cd04b5468a20e56cbb3052d07fff0400'

Can someone help me decode this so I can figure out where this event
originates, or (if this event is well-known to the folks working on
the trusted computing portion of systemd) tell me where this extension
is triggered in the source code? That will at least help me find and
hopefully understand the relevant change.

Thanks,
Kyle