linux-kernel - Re: [PATCH AUTOSEL 5.7 03/30] ima: extend boot

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Date:   Sun, 29 Nov 2020 08:17:38 -0500
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Sasha Levin <sashal@...nel.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Maurizio Drocco <maurizio.drocco@....com>,
        Bruno Meneguele <bmeneg@...hat.com>,
        linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org
Subject: Re: [PATCH AUTOSEL 5.7 03/30] ima: extend boot_aggregate with
 kernel measurements

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@....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.

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")