Re: [PATCH v6 7/7] tpm: pass an array of tpm_bank_list structures to tpm_pcr_extend()

[Roberto Sassu]

On 12/12/2018 7:27 PM, Jarkko Sakkinen wrote:
> On Thu, Dec 06, 2018 at 07:38:30PM +0100, Roberto Sassu wrote:
> > On 12/5/2018 1:14 AM, Jarkko Sakkinen wrote:
> > > On Tue, Dec 04, 2018 at 09:21:38AM +0100, Roberto Sassu wrote:
> > > > The new tpm_bank_list structure has been preferred to the tpm_digest
> > > > structure, to let the caller specify the size of the digest (which may be
> > > > unknown to the TPM driver).
> > >
> > > Why is that? Didn't previous commit query these?
> >
> > Since the TPM driver pads/truncates the first digest passed by the
> > caller to extend PCRs for which no digest was provided, it must know
> > which amount of data it can use. It is possible that the algorithm of
> > the first digest is unknown for the TPM driver, if the caller of
> > tpm_pcr_extend() didn't check chip->allocated_banks.
> >
> > By requiring that the caller passes also the digest size, this problem
> > does not arise. It seems reasonable to me to pass this information, as
> > the caller calculated the digest and it should know the digest size.
>
> OK. I noticed something other things that look to alarming:
>
> 1. The function does not fail if alg_id is not found. This will go
>     silent.

It is intentional. If alg_id is not found, the PCR is extended with the
first digest passed by the caller of tpm_pcr_extend(). If no digest was
provided, the PCR is extended with 0s. This is done to prevent that
PCRs in unused banks are extended later with fake measurements.


> 2. The function does not fail if there is a mismatch with the digest
>     sizes.

The data passed by the caller of tpm_pcr_extend() is copied to
dummy_hash, which has the maximum length. Then, tpm2_pcr_extend() takes
from dummy_hash as many bytes as needed, depending on the current
algorithm.

Roberto


> /Jarkko

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