IMA appraisal master plan?

[Mimi Zohar]

On Tue, 2017-11-21 at 10:33 +0100, Roberto Sassu wrote:
> On 11/20/2017 11:20 AM, Patrick Ohly wrote:
> > On Mon, 2017-11-20 at 07:47 +1100, James Morris wrote:
> >> On Fri, 17 Nov 2017, Roberto Sassu wrote:
> >>
> >>> LSMs are responsible to enforce a security policy at run-time,
> >>> while IMA/EVM protect data and metadata against offline attacks.
> >>
> >> In my view, IMA can also protect against making an online attack
> >> persistent across boots, and that would be the most compelling use of
> >> it for many general purpose applications.
> 
> It would be possible, if IMA knows when the system is in the expected
> state. For example, if the system is in the expected state after digest
> lists have been loaded, IMA could erase the EVM key, sealed to that
> state, when a file with unknown digest is measured. The system won't be
> able to produce valid HMACs, and files modified after the attack can be
> identified at the next boot, due to the invalid HMAC. Also accessing
> files with invalid HMAC will cause the EVM key to be zeroed.

Roberto, allowing the system to boot with an EVM HMAC key, but then
transition to a point when it can't be used, is a good idea.  The
transitioning, however, shouldn't be tied to white lists.  Please keep
these concepts independent of each other.

Preventing a device from booting is major.  Is there a less drastic
solution that would allow detection, without resealing the EVM HMAC
key so it can't be used?

Years ago Dave and I had a prototype of "locking" mutable files, after
a certain point in the boot process, working.  It allowed the ~20
mutable files to be created/updated, as necessary.  The limitation was
that any package updates or new packages installations needed to be
done during this window, before the transition, as well.

Mimi

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