linux-kernel - Re: [PATCH v5 6/7] tpm: ensure that the output of PCR read contains the correct digest size

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Date:   Mon, 19 Nov 2018 18:07:02 +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,
        stable@...r.kernel.org
Subject: Re: [PATCH v5 6/7] tpm: ensure that the output of PCR read contains
 the correct digest size

On Mon, Nov 19, 2018 at 04:09:34PM +0100, Roberto Sassu wrote:
> On 11/19/2018 3:33 PM, Jarkko Sakkinen wrote:
> > On Mon, Nov 19, 2018 at 09:14:00AM +0100, Roberto Sassu wrote:
> > > On 11/18/2018 8:32 AM, Jarkko Sakkinen wrote:
> > > > On Fri, Nov 16, 2018 at 05:06:48PM +0100, Roberto Sassu wrote:
> > > > > On 11/16/2018 2:41 PM, Jarkko Sakkinen wrote:
> > > > > > On Wed, Nov 14, 2018 at 04:31:07PM +0100, Roberto Sassu wrote:
> > > > > > > This patch protects against data corruption that could happen in the bus,
> > > > > > > by checking that that the digest size returned by the TPM during a PCR read
> > > > > > > matches the size of the algorithm passed to tpm2_pcr_read().
> > > > > > > 
> > > > > > > This check is performed after information about the PCR banks has been
> > > > > > > retrieved.
> > > > > > > 
> > > > > > > Signed-off-by: Roberto Sassu <roberto.sassu@...wei.com>
> > > > > > > Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
> > > > > > > Cc: stable@...r.kernel.org
> > > > > > 
> > > > > > Missing fixes tag.
> > > > > 
> > > > > Before this patch set, tpm2_pcr_extend() always copied 20 bytes from the
> > > > > output sent by the TPM.
> > > > > 
> > > > > Roberto
> > > > 
> > > > Aah, right, of course. Well the patch set is ATM somewhat broken because
> > > > this would require a fixes tag that points to a patch insdie the patch
> > > > set.
> > > > 
> > > > Probably good way to fix the issue is to just merge this with the
> > > > earlier commit.
> > > 
> > > Unfortunately, it is not possible. The exact digest size has been
> > > introduced with patch 5/7.
> > 
> > I put this in simple terms: if I merge 5/7 the driver will be broken.
> > Any commit in the patch set should not leave the tree in broken state.
> 
> Patch 6/7 does not fix patch 5/7. The check in patch 6/7 is not done for
> the PCR read performed by patch 5/7, because the digest sizes are not
> yet available.

Ah, right. Point taken :-)

/Jarkko