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

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Message-ID: <04dc04872f2925166f969b43852161d468ee899a.camel@linux.ibm.com>
Date: Mon, 14 Oct 2024 07:45:57 -0400
From: Mimi Zohar <zohar@...ux.ibm.com>
To: Jarkko Sakkinen <jarkko@...nel.org>,
        Roberto Sassu
	 <roberto.sassu@...weicloud.com>,
        linux-integrity@...r.kernel.org
Cc: James.Bottomley@...senPartnership.com, roberto.sassu@...wei.com,
        mapengyu@...il.com, David Howells <dhowells@...hat.com>,
        Paul Moore
 <paul@...l-moore.com>, James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn"
 <serge@...lyn.com>, Peter Huewe <peterhuewe@....de>,
        Jason Gunthorpe
 <jgg@...pe.ca>, keyrings@...r.kernel.org,
        linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 0/5] Lazy flush for the auth session

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