linux-kernel - Re: [PATCH v4 1/6] tpm: dynamically allocate active

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Date:   Thu, 8 Nov 2018 17:21:24 +0200
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Nayna Jain <nayna@...ux.ibm.com>
Cc:     Roberto Sassu <roberto.sassu@...wei.com>, zohar@...ux.ibm.com,
        linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, silviu.vlasceanu@...wei.com,
        Ken Goldman <kgold@...ux.vnet.ibm.com>,
        Kenneth Goldman <kgoldman@...ibm.com>
Subject: Re: [PATCH v4 1/6] tpm: dynamically allocate active_banks array

On Thu, Nov 08, 2018 at 07:20:51PM +0530, Nayna Jain wrote:
> Based on a discussion with Ken, the count in the TPML_PCR_SELECTION returns
> the number of possible algorithms supported. In the example below, two
> possible algorithms - SHA1 and SHA256 - are returned.
> 
> # /usr/local/bin/tssgetcapability -cap 5
> 2 PCR selections
>     hash TPM_ALG_SHA1
>     TPMS_PCR_SELECTION length 3
>     ff ff ff
>     hash TPM_ALG_SHA256
>     TPMS_PCR_SELECTION length 3
>     00 00 00
> 
> The pcr_select fields - "ff ff ff" and "00 00 00" - are bit masks for the
> enabled PCRs. The SHA1 bank is enabled for all PCRs (0-23), while the SHA256
> bank is not enabled.
> 
> The current code works, but it unnecessarily extends some banks. Instead of
> basing the number of active banks on the number of algorithms returned, it
> should be based on the pcr_select field.
> 
>    - Mimi & Nayna

I would just allocate array of the size of possible banks and grow
nr_active_banks for active algorithms to keep the code simple because
we are talking about insignificant amount of wasted space (might be
even zero bytes given how kernel allocators works)>

/Jarkko