On 2020-12-11 06:01:54, Mimi Zohar wrote:

On Thu, 2020-12-10 at 21:10 -0600, Tyler Hicks wrote:

quoted

On 2020-11-29 08:17:38, Mimi Zohar wrote:

quoted

Hi Sasha,

On Wed, 2020-07-08 at 21:27 -0400, Sasha Levin wrote:

quoted

On Wed, Jul 08, 2020 at 12:13:13PM -0400, Mimi Zohar wrote:

quoted

Hi Sasha,

On Wed, 2020-07-08 at 11:40 -0400, Sasha Levin wrote:

quoted

From: Maurizio Drocco <redacted>

[ 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 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