Re: [Linux-ima-devel] [PATCH v3 0/6] Updated API for TPM 2.0 PCR extend

Mimi Zohar
2017-06-28T22:29:19Z

On Wed, 2017-06-28 at 20:28 +0300, Jarkko Sakkinen wrote:
> On Mon, Jun 26, 2017 at 08:33:59AM -0400, Mimi Zohar wrote:
> > On Sat, 2017-06-24 at 11:03 +0200, Jarkko Sakkinen wrote:
> > > On Wed, Jun 21, 2017 at 04:29:35PM +0200, Roberto Sassu wrote:
> >
> >
> > > To move this forward and be more constructive here's how I see it
> > > should be done (along the lines, draft):
> > >
> > > int tpm_pcr_extend(u32 chip_num, int pcr_idx, unsigned int alg,
> > > const u8 *hash);
> > >
> > > The paramater 'alg' is crypto ID as specified by crypto subsystem.
> >
> > Based on Kenneth Goldman's input, the new IMA TPM-2.0 crypto hash
> > agile measurement list will contain the TPM crypto hash algorithm ids
> > (TPM crypto-ID).
>
> Doesn't this lock you to TPM?
>
> If you seriously want to do this, I guess it is fine by me but I'm just
> wondering why the measurement list couldn't use something with more
> loose binding to TPM.

I expect Ken will comment ...

> > > TPM driver must have a precompiled table of mappings for crypto IDs
> > > and TPM algorithm IDs.
> >
> > We could map the TPM crypto-IDs to the crypto subsystem IDs and then
> > map them back, but is that necessary?
> >
> > >
> > > In addition it must have dynamically acquired list of TPM alg IDs.
> > > For those algs that static mapping does not exist it must extend
> > > them like we do now everything else except SHA-1 (Naynas changes).
> >
> > Padding/truncating an unknown bank using SHA1 is fine, but at some
> > point, as Roberto pointed out to me, TPM 2.0's might not support SHA-
> > 1.  So for the record, we're hard coding the use of SHA1 for the
> > unknown algorithms whether or not the TPM supports SHA1.
>
> Why doesn't it work to pick algorithm X from the availabe options and
> do truncation/padding for that? Not necessarily SHA1.

Yes, it does work, as Roberto pointed out in a subsequent post.  For
TPM 2.0 the first digest algorithm in the IMA hash agile crypto
header, will be used as the default digest used for truncating/padding
the other unspecified banks.

In order not to break the existing userspace ABI, we will still need
to support the existing SHA1 based IMA securityfs measurement lists,
whether or not SHA1 is included in the hash agile IMA securityfs
measurement lists.

> >
> > > There's absolutely no need to pass digest size like you do BTW as it is
> > > defined by the standard.
> >
> > For algorithms known to the crypto subsystem, that is fine, but for
> > the unknown TPM crypto algorithms, we would need to somehow query the
> > TPM for the digest sizes to create the mapping.
> >
>
> There's a TPM command to query TPM algorithms.

Right, tpm2_init_pcr_bank_info(), defined in Roberto's patch "tpm:
introduce tpm_pcr_bank_info structure with digest_size from TPM", gets
the TPM digest size and stores it in the active bank structure
(tpm_pcr_bank_info).

Mimi