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

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Date:   Thu, 8 Nov 2018 20:57:29 +0200
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Mimi Zohar <zohar@...ux.ibm.com>
Cc:     Nayna Jain <nayna@...ux.ibm.com>,
        Roberto Sassu <roberto.sassu@...wei.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 10:29:53AM -0500, Mimi Zohar wrote:
> On Thu, 2018-11-08 at 17:21 +0200, Jarkko Sakkinen wrote:
> > 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)>
> 
> That's fine.  Remember the memory is just one concern, but the other
> concerns are the performance of calculating the unneeded hash and the
> TPM performance of including it in the PCR extend.

The driver would initialize only as many entries as are active array and
set nr_active_banks accordingly.

/Jarkko