Re: [PATCH v5 0/5] Lazy flush for the auth session

[Mimi Zohar]

On Sat, 2024-10-12 at 13:56 +0300, Jarkko Sakkinen wrote:
> On Fri, 2024-10-11 at 19:25 +0300, Jarkko Sakkinen wrote:
> > On Fri, 2024-10-11 at 18:10 +0200, Roberto Sassu wrote:
> > > Initially, I thought that maybe it would not be good to have an
> > > event
> > > log with unmodified and altered measurement entries. Then, I tried
> > > to
> > > think if we can really prevent an active interposer from injecting
> > > arbitrary PCR extends and pretending that those events actually
> > > happened.
> > > 
> > > If I understood James's cover letter correctly, the kernel can
> > > detect
> > > whether a TPM reset occurred, but not that a PCR extend occurred
> > > (maybe
> > > with a shadow PCR?).
> > 
> > We can detect TPM reset indirectly. I.e. null seed re-randomizes
> > per reset.
> > 
> > > 
> > > Second point, do we really want to take the responsibility to
> > > disable
> > > the protection on behalf of users? Maybe a better choice is to let
> > > them
> > > consciously disable HMAC protection.
> > 
> > So when IMA is not used already with these fixes we get good
> > results. And for tpm2_get_random() we can make the algorithm
> > smarter. All in all we have good path ongoing for "desktop
> > use case" that I would keep thing way there are or at least
> > postpone any major decisions just a bit.
> > 
> > For server/IMA use case I'll add a boot parameter it can be
> > either on or off by default, I will state that in the commit
> > message and we'll go from there.

Sounds good.

> 
> Up until legit fixes are place distributors can easily disable
> the feature. It would be worse if TCG_TPM2_HMAC did not exist.
> 
> So I think it is better to focus on doing right things right,
> since the feature itself is useful objectively, and make sure
> that those fixes bring the wanted results.

Are you backtracking on having a boot parameter here specifically to turn on/off
HMAC encryption for IMA?

Mimi