linux-kernel - Re: [PATCH v2 1/5] tpm: introduce tpm_pcr

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Message-ID: <8aacf9c7-ac99-578e-ca60-6bb45bf3ab2b@huawei.com>
Date:   Wed, 17 May 2017 10:42:35 +0200
From:   Roberto Sassu <roberto.sassu@...wei.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
CC:     <tpmdd-devel@...ts.sourceforge.net>,
        <linux-ima-devel@...ts.sourceforge.net>,
        <linux-security-module@...r.kernel.org>,
        <keyrings@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/5] tpm: introduce tpm_pcr_algorithms()

On 5/15/2017 3:18 PM, Roberto Sassu wrote:
>
>
> On 5/15/2017 12:36 PM, Jarkko Sakkinen wrote:
>> On Fri, May 05, 2017 at 04:21:48PM +0200, Roberto Sassu wrote:
>>> This function allows TPM users to know which algorithms the TPM
>>> supports.
>>> It stores the algorithms in a static array of 'enum tpm2_algorithms',
>>> allocated by the caller. If the array is not large enough, the function
>>> returns an error. Otherwise, it returns the number of algorithms written
>>> to the array. If the TPM version is 1.2, the function writes
>>> TPM2_ALG_SHA1
>>> to first element of the array.
>>>
>>> Writing the algorithm also for TPM 1.2 has the advantage that callers
>>> can use the API, tpm_pcr_algorithms() and tpm_pcr_extend(), regardless
>>> of the TPM version.
>>>
>>> A minor change added to this patch was to make available the size of
>>> the active_banks array, member of the tpm_chip structure, outside
>>> the TPM driver. The array size (TPM_ACTIVE_BANKS_MAX) has been exported
>>> to include/linux/tpm.h.
>>>
>>> With this information, callers of tpm_pcr_algorithms() can provide
>>> a static array with enough room for all the algorithms, instead
>>> of receiving the pointer of a dynamic array that they have to free
>>> later.
>>>
>>> Signed-off-by: Roberto Sassu <roberto.sassu@...wei.com>
>>> ---
>>> v2
>>>
>>> - tpm_pcr_algorithms() returns supported algorithms also for TPM 1.2
>>>
>>>  drivers/char/tpm/tpm-interface.c | 46
>>> ++++++++++++++++++++++++++++++++++++++++
>>>  drivers/char/tpm/tpm.h           | 13 +-----------
>>>  include/linux/tpm.h              | 19 +++++++++++++++++
>>>  3 files changed, 66 insertions(+), 12 deletions(-)
>>>
>>> diff --git a/drivers/char/tpm/tpm-interface.c
>>> b/drivers/char/tpm/tpm-interface.c
>>> index 4ed08ab..b90de3d 100644
>>> --- a/drivers/char/tpm/tpm-interface.c
>>> +++ b/drivers/char/tpm/tpm-interface.c
>>> @@ -911,6 +911,52 @@ int tpm_pcr_extend(u32 chip_num, int pcr_idx,
>>> const u8 *hash)
>>>  EXPORT_SYMBOL_GPL(tpm_pcr_extend);
>>>
>>>  /**
>>> + * tpm_pcr_algorithms - get TPM IDs of active PCR banks algorithms
>>
>> The grammar is incorrect here I believe. Should rather be
>>
>>   "algorithms of the active PCR banks"
>>
>> And there is no such thing as "TPM ID".
>>
>>> + * @chip_num:    tpm idx # or ANY
>>> + * @count:    # of items in algorithms
>>> + * @algorithms:    array of TPM IDs
>>> + *
>>> + * Returns < 0 on error, and the number of active PCR banks on success.
>>> + *
>>> + * TPM 1.2 has always one SHA1 bank.
>>> + */
>>> +int tpm_pcr_algorithms(u32 chip_num, int count,
>>> +               enum tpm2_algorithms *algorithms)
>>                        unsigned int
>>
>> In addition the function name is not too greatg,
>>
>> Your syntax for return value is not correct. In addition after
>> describing the return value there should not be anything. You should
>> study
>>
>> https://www.kernel.org/doc/Documentation/kernel-doc-nano-HOWTO.txt
>>
>> Better name for the function would be tpm_get_pcr_algorithms().
>>
>>> +{
>>> +    struct tpm_chip *chip;
>>> +    int i;
>>> +
>>> +    if (count <= 0 || algorithms == NULL)
>>> +        return -EINVAL;
>>
>> Is there a legal case where caller would pass these values? Now it
>> looks like that there is.
>>
>> 'count' should unsigned int and zero should be a legal value for
>> count.
>
> I wanted to avoid that a negative value returned by tpm_pcr_algorithms()
> is passed to tpm_pcr_extend() as unsigned int.
>
>
>> That said I think the whole design is wrong as you could calculate
>> array for algs only one time and pass a const reference to it on
>> request.
>
> Ok. If I understood it correctly, you are saying to pass a const
> reference of chip->active_banks. Then, I should also:

This is not a viable option. chip could be freed and the reference
becomes invalid, without increasing the reference count.

Did you think about something different?

Thanks

Roberto