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

[Roberto Sassu <roberto.sassu-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>]

On 5/24/2017 7:35 PM, Jarkko Sakkinen wrote:
> On Mon, May 22, 2017 at 11:07:54AM +0200, Roberto Sassu wrote:
>> 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-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>
>>>>>>> ---
>>>>>>> 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).
>
> What is VLAI?

Variable-length arrays in structures.


> You can always allocate it from heap.

The algorithms to be used by IMA are set in a __setup function.


>> 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
>
> I don't understand what you mean.

I meant that callers of tpm_pcr_algorithms() can always pass
an array whose size is the same of that of tpm_chip->active_banks.
Since the array size is known, passing count is not necessary.

Roberto

------------------------------------------------------------------------------
Check out the vibrant tech community on one of the world's most
engaging tech sites, Slashdot.org! http://sdm.link/slashdot