Re: [PATCH v4 1/1] tpm: add sysfs exports for all banks of PCR registers

[Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>]

> On Tue, 2020-08-18 at 14:17 -0300, Jason Gunthorpe wrote:
> > On Tue, Aug 18, 2020 at 09:44:30AM -0700, James Bottomley wrote:
> > 
> > > The question you should be asking isn't whether the information
> > > *could*
> > > be obtained by other means, but whether providing it in this form
> > > facilitates current operations and whether the interface would have
> > > users.
> > 
> > Sure. What are the use cases that need PCRs but no other TPM
> > operations?
> > 
> > The cover letter didn't say. As PCR is really only useful in the
> > context of the local TPM I'm not thinking of anything..
> 
> The three use cases I picked up at the Boot and Security MC were:
> 
>    1. local log verification: a script can run through the IMA ascii log
>       and construct the PCR 10 hash which can then be verified
>    2. Knowing what the PCR values actually are for sealed keys.  With the
>       current trusted key infrastructure you have to calculate and supply
>       the hash yourself.  With the new proposed infrastructure, the hash
>       would be calculated by the seal operation, but you're still going to
>       need the actual PCR values to debug unseal failures.
>    3. As a stability check for log shipping: you read the PCR take the log
>       then read the PCR: if the two reads are the same the PCR backing the
>       log is stable for quoting.
> 
> James

The proposed sysfs attributes are racy in the sense that PCRs could
change in-between reading different hashes.

A blob containing all the hashes would make more sense as it does not
have this issue.

If this is for scripts to further process, it is also more efficient
than printable ASCII text.

/Jarkko