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

[Roberto Sassu]

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@xxxxxxxxxx>
> > > > ---
> > > > 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