Re: [PATCH v2 2/5] tpm: introduce tpm_pcr_algo_to_crypto() and tpm_pcr_algo_from_crypto()

Roberto Sassu
2017-05-30T10:24:47Z

On 5/24/2017 10:25 PM, Jarkko Sakkinen wrote:
> On Wed, May 24, 2017 at 10:33:54AM -0700, Jarkko Sakkinen wrote:
>> On Mon, May 22, 2017 at 09:21:28AM +0200, Roberto Sassu wrote:
>>> On 5/20/2017 3:22 PM, Jarkko Sakkinen wrote:
>>>> On Mon, May 15, 2017 at 04:22:22PM +0200, Roberto Sassu wrote:
>>>>> On 5/15/2017 1:16 PM, Jarkko Sakkinen wrote:
>>>>>> On Fri, May 05, 2017 at 04:21:49PM +0200, Roberto Sassu wrote:
>>>>>>> tpm_pcr_algorithms() returns to its callers the IDs of the hash algorithms
>>>>>>> supported by the TPM. This patch introduces tpm_pcr_algo_to_crypto(),
>>>>>>> so that the callers can use the crypto subsystem to calculate the digest
>>>>>>> to be passed to tpm_pcr_extend().
>>>>>>>
>>>>>>> tpm_pcr_algo_from_crypto(), implemented for completeness, is instead used
>>>>>>
>>>>>> What do you mean by completeness? Please, never add unused stuff.
>>>>>>
>>>>>>> by tpm2_seal_trusted() to perform the opposite conversion.
>>>>>>>
>>>>>>> Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
>>>>>>> ---
>>>>>>> v2
>>>>>>>
>>>>>>> - fixed return values of tpm2_pcr_algo_to_crypto() and
>>>>>>> tpm2_pcr_algo_from_crypto() if TPM support is disabled in the kernel
>>>>>>
>>>>>> Change Log only to the cover letter.
>>>>>>
>>>>>>> drivers/char/tpm/tpm-interface.c | 51 ++++++++++++++++++++++++++++++++++++++++
>>>>>>> drivers/char/tpm/tpm2-cmd.c | 42 +++++++++------------------------
>>>>>>> include/linux/tpm.h | 13 ++++++++++
>>>>>>> 3 files changed, 75 insertions(+), 31 deletions(-)
>>>>>>
>>>>>> This commit is just deadly wrong in so many ways.
>>>>>>
>>>>>> I would suggest to make extend always just take crypto ID in so you
>>>>>> don't have to add these bizarre conversion functions.
>>>>>
>>>>> The reason of this choice (as I explained in the cover letter)
>>>>> is that TPM users might want to produce an event log with
>>>>> the TCG format (which includes the TPM algorithm ID). Also,
>>>>> TPM IDs should be preferred because, with them, TPM users
>>>>> can calculate a digest directly with the TPM.
>>>>>
>>>>> Taking crypto IDs means relying on the fact that there
>>>>> is always a mapping between TPM IDs and crypto IDs.
>>>>> Otherwise, tpm_pcr_algorithms() cannot return the algorithms
>>>>> to its callers and PCRs cannot be extended. If TPM IDs are used,
>>>>> TPM users have two alternatives: calculate the digest with
>>>>> the TPM, or pass a SHA1 digest to tpm_pcr_extend(), so that
>>>>> it can be padded to extend remanining PCR banks.
>>>>>
>>>>> However, this second option will work only when the TPM driver
>>>>> determines the size of an algorithm without relying on the crypto
>>>>> subsystem. At the moment, if a TPM ID is not mapped to a crypto ID,
>>>>> tpm2_pcr_extend() ignores the digest.
>>>>>
>>>>> Roberto
>>>>
>>>> What if tpm_pcr_algorithms would convert TPM IDs to crypto IDs?
>>>>
>>>> Externally other subsystems would have to then deal with only crypto
>>>> IDs.
>>>
>>> Then, other subsystems cannot:
>>>
>>> - use the TPM to calculate a digest
>>> - create an event log with the TCG format
>>>
>>> Roberto
>>
>> Can you open this up a bit? What is the use case that you canno
>> implement?
>>
>> /Jarkko
>
> The most important question is: are this used *right now*. If not, it is
> a definitive NAK. We don't want any future-proof code to the kernel.
> Always do the lowest common denominator in terms of generality and we
> will refactor from there when need arises.

This is still being discussed.

If we use crypto IDs, as you suggested, what it should be passed
to tpm_pcr_extend()? Or, probably to a new function (Mimi said that
changing the parameters of an existing function is not bisect-safe).

Roberto

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