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

Tyler Hicks
2020-12-11T03:11:50Z

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.

Tyler

>
> Any place commit 6f1a1d103b48 was backported [2], this commit
> 20c59ce010f8 ("ima: extend boot_aggregate with kernel measurements")
> should be backported as well.
>
> thanks,
>
> Mimi
>
> [1] commit 1ea973df6e21 ("ima: Calculate and extend PCR with digests in ima_template_entry")
> [2] commit 6f1a1d103b48 ("ima: Switch to ima_hash_algo for boot aggregate")
>