Re: [PATCH AUTOSEL 5.7 03/30] ima: extend boot_aggregate with kernel measurements

Mimi Zohar
2020-12-11T11:03:28Z

On Thu, 2020-12-10 at 21:10 -0600, Tyler Hicks wrote:
> On 2020-11-29 08:17:38, Mimi Zohar wrote:
> > Hi Sasha,
> >
> > On Wed, 2020-07-08 at 21:27 -0400, Sasha Levin wrote:
> > > On Wed, Jul 08, 2020 at 12:13:13PM -0400, Mimi Zohar wrote:
> > > >Hi Sasha,
> > > >
> > > >On Wed, 2020-07-08 at 11:40 -0400, Sasha Levin wrote:
> > > >> From: Maurizio Drocco <maurizio.drocco@ibm.com>
> > > >>
> > > >> [ Upstream commit 20c59ce010f84300f6c655d32db2610d3433f85c ]
> > > >>
> > > >> Registers 8-9 are used to store measurements of the kernel and its
> > > >> command line (e.g., grub2 bootloader with tpm module enabled). IMA
> > > >> should include them in the boot aggregate. Registers 8-9 should be
> > > >> only included in non-SHA1 digests to avoid ambiguity.
> > > >
> > > >Prior to Linux 5.8, the SHA1 template data hashes were padded before
> > > >being extended into the TPM. Support for calculating and extending
> > > >the per TPM bank template data digests is only being upstreamed in
> > > >Linux 5.8.
> > > >
> > > >How will attestation servers know whether to include PCRs 8 & 9 in the
> > > >the boot_aggregate calculation? Now, there is a direct relationship
> > > >between the template data SHA1 padded digest not including PCRs 8 & 9,
> > > >and the new per TPM bank template data digest including them.
> > >
> > > Got it, I'll drop it then, thank you!
> >
> > After re-thinking this over, I realized that the attestation server can
> > verify the "boot_aggregate" based on the quoted PCRs without knowing
> > whether padded SHA1 hashes or per TPM bank hash values were extended
> > into the TPM[1], but non-SHA1 boot aggregate values [2] should always
> > include PCRs 8 & 9.
>
> I'm still not clear on how an attestation server would know to include
> PCRs 8 and 9 after this change came through a stable kernel update. It
> doesn't seem like something appropriate for stable since it requires
> code changes to attestation servers to handle the change.
>
> I know this has already been released in some stable releases, so I'm
> too late, but perhaps I'm missing something.

The point of adding PCRs 8 & 9 only to non-SHA1 boot_aggregate values
was to avoid affecting existing attestation servers. The intention was
when attestation servers added support for the non-sha1 boot_aggregate
values, they'd also include PCRs 8 & 9. The existing SHA1
boot_aggregate value remains PCRs 0 - 7.

To prevent this or something similar from happening again, what should
have been the proper way of including PCRs 8 & 9?

thanks,

Mimi