linux-kernel - Re: [PATCH] extend IMA boot_aggregate with kernel measurements

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Date:   Fri, 12 Jun 2020 10:14:19 -0700
From:   James Bottomley <jejb@...ux.ibm.com>
To:     Roberto Sassu <roberto.sassu@...wei.com>,
        Maurizio Drocco <maurizio.drocco@....com>,
        "zohar@...ux.ibm.com" <zohar@...ux.ibm.com>
Cc:     "dmitry.kasatkin@...il.com" <dmitry.kasatkin@...il.com>,
        "jmorris@...ei.org" <jmorris@...ei.org>,
        "linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>,
        "serge@...lyn.com" <serge@...lyn.com>,
        Silviu Vlasceanu <Silviu.Vlasceanu@...wei.com>
Subject: Re: [PATCH] extend IMA boot_aggregate with kernel measurements

On Fri, 2020-06-12 at 15:11 +0000, Roberto Sassu wrote:
> with recent patches, boot_aggregate can be calculated from non-SHA1
> PCR banks. I would replace with:
> 
> Extend cumulative digest over ...
> 
> Given that with this patch boot_aggregate is calculated differently,
> shouldn't we call it boot_aggregate_v2 and enable it with a new
> option?

So here's the problem: if your current grub doesn't do any TPM
extensions (as most don't), then the two boot aggregates are the same
because PCRs 8 and 9 are zero and there's a test that doesn't add them
to the aggregate if they are zero.  For these people its a nop so we
shouldn't force them to choose a different version of the same thing.

If, however, you're on a distribution where grub is automatically
measuring the kernel and command line into PCRs 8 and 9 (I think Fedora
32 does this), your boot aggregate will change.  It strikes me in that
case we can call this a bug fix, since the boot aggregate isn't
properly binding to the previous measurements without PCRs 8 and 9.  In
this case, do we want to allow people to select an option which doesn't
properly bind the IMA log to the boot measurements?  That sounds like a
security hole to me.

However, since it causes a user visible difference in the grub already
measures case, do you have a current use case that would be affected? 
As in are lots of people already running a distro with the TPM grub
updates and relying on the old boot aggregate?

James