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

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Message-ID: <ed2d769b-2917-7d8b-ff9c-16220fa17833@huawei.com>
Date:   Mon, 19 Nov 2018 16:09:34 +0100
From:   Roberto Sassu <roberto.sassu@...wei.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.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 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.

Roberto


> That implies that 5/7 and 6/7 cannot be separate commits.
> 
> /Jarkko
> 

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