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

Jarkko Sakkinen
2017-05-20T13:22:27Z

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.

That would favor "memcpy" (or it would become a for loop) for 1/5.

/Jarkko