linux-kernel - Re: [tpmdd-devel] [PATCH v2 0/2] tpm: enhance TPM 2.0 extend function to support multiple PCR banks

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Date:   Tue, 3 Jan 2017 15:29:43 +0200
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Nayna Jain <nayna@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        tpmdd-devel@...ts.sourceforge.net
Subject: Re: [tpmdd-devel] [PATCH v2 0/2] tpm: enhance TPM 2.0 extend
 function to support multiple PCR banks

On Tue, Jan 03, 2017 at 12:15:50AM +0200, Jarkko Sakkinen wrote:
> On Fri, Dec 30, 2016 at 02:02:28PM -0500, Nayna Jain wrote:
> > IMA extends its hash measurements in the TPM PCRs, based on policy.
> > The existing in-kernel TPM extend function extends only the SHA1
> > PCR bank. TPM 2.0 defines multiple PCR banks, to support different
> > hash algorithms. The TCG TPM 2.0 Specification[1] recommends
> > extending all active PCR banks to prevent malicious users from
> > setting unused PCR banks with fake measurements and quoting them.
> > This patch set adds support for extending all active PCR banks,
> > as recommended.
> > 
> > The first patch implements the TPM 2.0 capability to retrieve
> > the list of active PCR banks.
> > 
> > The second patch modifies the tpm_pcr_extend() and tpm2_pcr_extend()
> > interface to support extending multiple PCR banks. The existing
> > tpm_pcr_extend() interface expects only a SHA1 digest. Hence, to
> > extend all active PCR banks with differing digest sizes for TPM 2.0,
> > the SHA1 digest is padded with 0's as needed.
> > 
> > This approach is taken to maintain backwards compatibility for IMA
> > in order to continue working with both TPM 1.2 and TPM 2.0 without
> > any changes and still comply with TCG TPM 2.0 Specification[1].
> 
> What is the plan to improve IMA so that it can use better hash
> algorithms? For me this zero padding sounds like a hack.

I'm fine with zero padding if there is also at minimum to improve the
situation. I do not want to apply these patches if they are ought to
become a bottlenek.

/Jarkko