Re: [PATCH v5 7/7] tpm: pass an array of tpm_bank_list structures to tpm_pcr_extend()

[Mimi Zohar]

On Mon, 2018-11-19 at 09:16 +0100, Roberto Sassu wrote:
> On 11/19/2018 5:57 AM, Mimi Zohar wrote:
> > On Sun, 2018-11-18 at 09:27 +0200, Jarkko Sakkinen wrote:
> >> On Fri, Nov 16, 2018 at 04:55:36PM +0100, Roberto Sassu wrote:
> >>> On 11/16/2018 4:03 PM, Jarkko Sakkinen wrote:
> >>>> On Wed, Nov 14, 2018 at 04:31:08PM +0100, Roberto Sassu wrote:
> >>>>> Currently, tpm_pcr_extend() accepts as an input only a SHA1 digest.
> >>>>>
> >>>>> This patch modifies the definition of tpm_pcr_extend() to allow other
> >>>>> kernel subsystems to pass a digest for each algorithm supported by the TPM.
> >>>>> All digests are processed by the TPM in one operation.
> >>>>>
> >>>>> If a tpm_pcr_extend() caller provides a subset of the supported algorithms,
> >>>>> the TPM driver extends the remaining PCR banks with the first digest
> >>>>> passed as an argument to the function.
> >>>>
> >>>> What is the legit use case for this?
> >>>
> >>> A subset could be chosen for better performance, or when a TPM algorithm
> >>> is not supported by the crypto subsystem.
> >>
> >> Doesn't extending a subset a security concern?
> > 
> > Right, so instead of extending a subset of the allocated banks, all of
> > the allocated banks need to be extended, even for those banks that a
> > digest was not included. ÂThis is no different than what is being done
> > today. ÂIMA is currently only calculating the SHA1 hash, padding the
> > digest with 0's, and extending the padded value(s) into all of the
> > allocated banks.
> 
> The caller of tpm_pcr_extend() could pass a subset of the allocated
> banks, but the TPM driver extends all banks as before.

Agreed, there should be a clear division.

1) The caller shouldn't need to know anything about the chip->info.
2) The TPM driver should not rely on the caller to supply all the
hashes, but verify that all allocated banks are being extended.

Mimi

> 
> 
> > If there is a vulnerability with the hash algorithm, then any bank
> > extended with the padded/truncated digest would be susceptible.
> > 
> > IMA will need to become TPM 2.0 aware, calculating and extending
> > multiple banks and define a new measurement list format containing the
> > multiple digests.
> > 
> > Mimi
> > 
>