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

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Message-ID: <3a163a1839ff469acfa8dbb889c1b0889ec771bc.camel@linux.ibm.com>
Date:   Tue, 12 Jan 2021 11:56:45 -0500
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Tyler Hicks <tyhicks@...ux.microsoft.com>,
        Jerry Snitselaar <jsnitsel@...hat.com>
Cc:     Sasha Levin <sashal@...nel.org>, 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 Tyler,

On Tue, 2021-01-12 at 09:35 -0600, Tyler Hicks wrote:
> 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@....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?
> 
> > 
> > > 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.

As I recall, commit 6f1a1d103b48 ("ima: Switch to ima_hash_algo for
boot aggregate") was backported to address TPMs without SHA1 support,
as reported by Jerry.

Mimi