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

Mimi Zohar
2020-12-13T02:23:30Z

On Fri, 2020-12-11 at 09:46 -0800, James Bottomley wrote:
> On Fri, 2020-12-11 at 06:01 -0500, 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.
> >
> > To prevent this or something similar from happening again, what
> > should have been the proper way of including PCRs 8 & 9?
>
> Just to be pragmatic: this is going to happen again. Shim is already
> measuring the Mok variables through PCR 14, so if we want an accurate
> boot aggregate, we're going to have to include PCR 14 as well (or
> persuade shim to measure through a PCR we're already including, which
> isn't impossible since I think shim should be measuring the Mok
> variables using the EV_EFI_VARIABLE_DRIVER_CONFIG event and, since it
> affects secure boot policy, that does argue it should be measured
> through PCR 7).

Ok. Going forward, it sounds like we need to define a new
"boot_aggregate" record. One that contains a version number and PCR
mask.

Mimi