RE: [PATCH] extend IMA boot_aggregate with kernel measurements

[Roberto Sassu]

> From: James Bottomley [mailto:jejb@xxxxxxxxxxxxx]
> Sent: Friday, June 12, 2020 7:14 PM
> 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?

I don't know how many people would be affected. However, if an
attestation tool processes both measurement lists from unpatched kernels
and patched kernels, keeping the same name would be a problem as it
cannot be determined from the measurement list how boot_aggregate
was calculated.

Anyway, I agree this should be fixed. At least, I suggest to add a Fixes tag,
to ensure that this patch is applied to all stable kernels.

Roberto

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