linux-kernel - Re: [PATCH v6 7/7] tpm: pass an array of tpm_bank_list structures to tpm_pcr

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Date:   Fri, 14 Dec 2018 09:58:52 +0200
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Roberto Sassu <roberto.sassu@...wei.com>
Cc:     zohar@...ux.ibm.com, david.safford@...com, monty.wiseman@...com,
        linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, silviu.vlasceanu@...wei.com
Subject: Re: [PATCH v6 7/7] tpm: pass an array of tpm_bank_list structures to
 tpm_pcr_extend()

On Thu, Dec 13, 2018 at 08:57:17AM +0100, Roberto Sassu wrote:
> > 1. The function does not fail if alg_id is not found. This will go
> >     silent.
> 
> It is intentional. If alg_id is not found, the PCR is extended with the
> first digest passed by the caller of tpm_pcr_extend(). If no digest was
> provided, the PCR is extended with 0s. This is done to prevent that
> PCRs in unused banks are extended later with fake measurements.
> 
> 
> > 2. The function does not fail if there is a mismatch with the digest
> >     sizes.
> 
> The data passed by the caller of tpm_pcr_extend() is copied to
> dummy_hash, which has the maximum length. Then, tpm2_pcr_extend() takes
> from dummy_hash as many bytes as needed, depending on the current
> algorithm.

I would suggest to document these corner cases to the function long
description to make it easy and obvious to understand.

/Jarkko