Re: [PATCH v2 1/5] tpm: introduce tpm_pcr_algorithms()

Roberto Sassu
2017-05-22T09:08:38Z

On 5/20/2017 3:18 PM, Jarkko Sakkinen wrote:
> On Wed, May 17, 2017 at 10:42:35AM +0200, Roberto Sassu wrote:
>> 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@huawei.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
>
> Two alternatives come in mind.
>
> 1. add tpm_put to linclude/linux/tpm.h
> 2. take put_ops away from the implementation
> 3. add documentation that tpm_put needs to be called
>
> or acceptable alternative would memcpy in the implementation
>
> In any case, you should probably drop completely 'count' and have
> function for getting the number of active banks.

Having a variable number, makes things more complicated.
In the IMA patch set for multiple template digests,
the array of algorithms is stored inside a structure
called digest descriptor. The array size must be fixed,
to avoid VLAIS (which is not supported by clang).

Since the size of tpm_chip->active_banks is a small number,
we can drop count, and assume that the size of the array
passed to tpm_pcr_algorithms() always matches the size
of the active_banks array.

Roberto