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

Tyler Hicks
2021-01-12T15:37:17Z

On 2020-12-14 10:42:24, Tyler Hicks wrote:
> On 2020-12-11 06:01:54, Mimi Zohar wrote:
> > 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.
>
> AFAIK, there's nothing that prevents the non-SHA1 TPM 2.0 PCR banks from
> being used even before v5.8, albeit with zero padded SHA1 digests.
> Existing attestation servers that already support that configuration are
> broken by this stable backport.

To wrap up this thread, I think the last thing to address is if this
commit should be reverted from stable kernels? Do you have any thoughts
about that, Mimi?

Tyler

>
> > To prevent this or something similar from happening again, what should
> > have been the proper way of including PCRs 8 & 9?
>
> I don't think that commits like 6f1a1d103b48 ("ima: Switch to
> ima_hash_algo for boot aggregate") and 20c59ce010f8 ("ima: extend
> boot_aggregate with kernel measurements") should be backported to
> stable.
>
> Including PCRs 8 and 9 definitely makes sense to include in the
> boot_aggregate value but limiting such a change to "starting in 5.8",
> rather than "starting in 5.8 and 5.4.82", is the safer approach when
> attestation server modifications are required.
>
> Tyler
>
> >
> > thanks,
> >
> > Mimi
> >