Misleading command invocation nvram.md/sealing

[SharkOverBite]

The "Sealing" section of nvram.md demonstrates the following command sequence:

$ tpm2_pcrread -o srtm.pcrvalues sha256:0,1,2,3
$ tpm2_createpolicy --policy-pcr -l sha256:0,1,2,3 -f srtm.pcrvalues -L srtm.policy
$ tpm2_nvdefine 0x1500019 -C o -s 32 -L srtm.policy -a "policyread|policywrite"
$ tpm2_nvwrite 0x1500019 -C 0x1500019 -P pcr:sha256:0,1,2,3=srtm.pcrvalues -i testfile
$ tpm2_nvread 0x1500019 -C 0x1500019 -P pcr:sha256:0,1,2,3=srtm.pcrvalues 

You don't have to provide a file for tpm2_createpolicy, it will implicitly use the current values of the specified PCRs. But you show a more general case, and that's fine.

However, the inclusion of "=srtm.pcrvalues" in arguments for the last two tpm2_nvwrite and tpm2_nvread commands is totally bogus, and arguably should raise an error from the tpm2-tools parser. Obviously, when invoking those commands with a PCR policy, the contents of that are irrelevant. The source of the PCR values is (and must be) only the current state of the specified PCRs, on the device.

[iolivergithub]

Thanks,

the course and those commands are pretty old now. At least when I wrote it originally the above was the case.

I no longer work for Nokia, so I'm wondering what to do with that course --- I've been teaching it at universities (or actually using that material as part of the TPM part of my courses)....there's a book due out "real soon now"

t.

Ian

On Mon, 1 Jul 2024 at 03:11, SharkOverBite @.***> wrote:

The "Sealing" section of nvram.md demonstrates the following command sequence:

$ tpm2_pcrread -o srtm.pcrvalues sha256:0,1,2,3 $ tpm2_createpolicy --policy-pcr -l sha256:0,1,2,3 -f srtm.pcrvalues -L srtm.policy $ tpm2_nvdefine 0x1500019 -C o -s 32 -L srtm.policy -a "policyread|policywrite" $ tpm2_nvwrite 0x1500019 -C 0x1500019 -P pcr:sha256:0,1,2,3=srtm.pcrvalues -i testfile $ tpm2_nvread 0x1500019 -C 0x1500019 -P pcr:sha256:0,1,2,3=srtm.pcrvalues

You don't have to provide a file for tpm2_createpolicy, it will implicitly use the current values of the specified PCRs. But you show a more general case, and that's fine.

However, the inclusion of "=srtm.pcrvalues" in the last two command, to tpm2_nvwrite and tpm2_nvread is totally bogus, and arguably should raise an error from the tpm2-tools parser. Obviously, when invoking those commands with a PCR policy, this file is completely irrelevant. The source of the PCR values is (and must be) only the current state of the linked PCRs on the device.

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[SharkOverBite]

I understand. There's still, after all these years, a great lack of good, well-written (I can't bold that enough), procedural documentation about TPM. This one is better than most, though quite focused on attestation use-cases (which I care less about).

I have verified that the commands work without specifying the file (as logically, they must - what are the semantics of attaching the file??). Since I've opened an issue on tpm2-tools, we'll have to see what they have to say about it.

Good luck with the book (when and if). If it's well-written, I'd love to read it.

[iolivergithub]

Thanks,

yes, I certainly share your comment on a lack of procedural documentation. I worked extensively with attestation, hence the emphasis there ( even wrote the "best" attestation server ever ! )

I'll probably fork the course and rewrite it over the next few weeks - will go alongside the courses I teach and the upcoming "real soon now" book

t.

Ian

On Mon, 1 Jul 2024 at 18:11, SharkOverBite @.***> wrote:

I understand. There's still, after all these years, a great lack of good, well-written (I can't bold that enough), procedural documentation about TPM. This one is better than most, though quite focused on attestation use-cases (which I care less about).

I have verified that the commands work without specifying the file (as logically, they must, what are the semantics of attaching the file??). Since I've opened an issue on tpm2-tools, we'll have to see what they have to say about it.

Good luck with the book (when and if). If it's well-written, I'd love to read it.

— Reply to this email directly, view it on GitHub https://github.com/nokia/TPMCourse/issues/22#issuecomment-2200423690, or unsubscribe https://github.com/notifications/unsubscribe-auth/AHKY2MIQYKJ7L5V3DFMEGADZKFWR3AVCNFSM6AAAAABKEQZHDSVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDEMBQGQZDGNRZGA . You are receiving this because you commented.Message ID: @.***>

[SharkOverBite]

Looking forward to the fruits of your many labors. :+1:

[SharkOverBite]

tpm2-software/tpm2-tools say I'm wrong, that it's not an error but an optimization which supposedly saves another step by the device IIUC. I don't understand their response to be honest.