linux-kernel - Re: [PATCH v5 7/7] tpm: pass an array of tpm_bank_list structures to tpm_pcr

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Message-ID: <26519f33-b5d6-c2c6-5179-c43f197f2cbd@huawei.com>
Date:   Mon, 19 Nov 2018 09:16:45 +0100
From:   Roberto Sassu <roberto.sassu@...wei.com>
To:     Mimi Zohar <zohar@...ux.ibm.com>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
CC:     <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>
Subject: Re: [PATCH v5 7/7] tpm: pass an array of tpm_bank_list structures to
 tpm_pcr_extend()

On 11/19/2018 5:57 AM, Mimi Zohar wrote:
> On Sun, 2018-11-18 at 09:27 +0200, Jarkko Sakkinen wrote:
>> On Fri, Nov 16, 2018 at 04:55:36PM +0100, Roberto Sassu wrote:
>>> On 11/16/2018 4:03 PM, Jarkko Sakkinen wrote:
>>>> On Wed, Nov 14, 2018 at 04:31:08PM +0100, Roberto Sassu wrote:
>>>>> Currently, tpm_pcr_extend() accepts as an input only a SHA1 digest.
>>>>>
>>>>> This patch modifies the definition of tpm_pcr_extend() to allow other
>>>>> kernel subsystems to pass a digest for each algorithm supported by the TPM.
>>>>> All digests are processed by the TPM in one operation.
>>>>>
>>>>> If a tpm_pcr_extend() caller provides a subset of the supported algorithms,
>>>>> the TPM driver extends the remaining PCR banks with the first digest
>>>>> passed as an argument to the function.
>>>>
>>>> What is the legit use case for this?
>>>
>>> A subset could be chosen for better performance, or when a TPM algorithm
>>> is not supported by the crypto subsystem.
>>
>> Doesn't extending a subset a security concern?
> 
> Right, so instead of extending a subset of the allocated banks, all of
> the allocated banks need to be extended, even for those banks that a
> digest was not included.  This is no different than what is being done
> today.  IMA is currently only calculating the SHA1 hash, padding the
> digest with 0's, and extending the padded value(s) into all of the
> allocated banks.

The caller of tpm_pcr_extend() could pass a subset of the allocated
banks, but the TPM driver extends all banks as before.

Roberto


> If there is a vulnerability with the hash algorithm, then any bank
> extended with the padded/truncated digest would be susceptible.
> 
> IMA will need to become TPM 2.0 aware, calculating and extending
> multiple banks and define a new measurement list format containing the
> multiple digests.
> 
> Mimi
> 

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