Message-ID: <aOUkcytS21zQs71I@kernel.org>
Date: Tue, 7 Oct 2025 17:32:19 +0300
From: Jarkko Sakkinen <jarkko@...nel.org>
To: James Bottomley <James.Bottomley@...senpartnership.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Huewe <peterhuewe@....de>, Jason Gunthorpe <jgg@...pe.ca>,
	David Howells <dhowells@...hat.com>, keyrings@...r.kernel.org,
	linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [GIT PULL] TPM DEVICE DRIVER: tpmdd-next-v6.18

On Mon, Oct 06, 2025 at 07:57:10PM +0300, Jarkko Sakkinen wrote:
> On Mon, Oct 06, 2025 at 07:51:51PM +0300, Jarkko Sakkinen wrote:
> > On Mon, Oct 06, 2025 at 10:33:40AM -0400, James Bottomley wrote:
> > > On Mon, 2025-10-06 at 17:12 +0300, Jarkko Sakkinen wrote:
> > > > 2. Null seed was extremely bad idea. The way I'm planning to actually
> > > >    fix this is to parametrize the primary key to a persistent key
> > > > handle
> > > >    stored into nvram of the chip instead of genration. This will
> > > > address
> > > >    also ambiguity and can be linked directly to vendor ceritifcate
> > > >    for e.g. to perfom remote attesttion.
> > > 
> > > Just a minute, there's been no discussion or debate about this on the
> > > list.  The rationale for using the NULL seed is clearly laid out here:
> > > 
> > > https://docs.kernel.org/security/tpm/tpm-security.html
> > > 
> > > But in brief it is the only way to detect reset attacks against the TPM
> > > and a reset attack is the single simplest attack an interposer can do.
> > > 
> > > If you think there's a problem with the approach, by all means let's
> > > have a debate, since TPM security is always a trade off, but you can't
> > > simply come to your own opinion and try to impose it by fiat without at
> > > least raising whatever issue you think you've found with the parties
> > > who contributed the code in the first place.
> > 
> > Ok fair enough, it's quite context dependent what is not secure and
> > what is secure.
> > 
> > What I've thought, or have planned to implement, is not to discard null
> > seed but instead parmetrize the primary key as a kernel command-line
> > parameter.
> > 
> > E.g. "tpm.integrity_key={off,null,handle}" and
> > "tpm.integrity_key_handle" to specify an NV index. The default value is
> > off and I think also that with this change and possibly with some
> > additional polishing it can reappear in default config,
> > 
> > This out of context for the PR but I will take your comment into account
> > in the pull request.
> > 
> > My main issue preventing sending a new pull request is that weird list
> > of core TPM2 features that is claimed "not to be required" with zero
> > references. Especially it is contraditory claim that TPM2_CreatePrimary
> > would be optional feature as the whole chip standard is based on three
> > random seeds from which primary keys are templated and used as root
> > keys for other keys.
> > 
> > So I guess I cherry-pick the claims from Chris' patch that I can cope
> > with, look what I wrote to my commit and adjust that accordingly and
> > finally write a tag message with summarization of all this. I exactly
> > drop the arguments with no quantitative evidence, which is probably
> > a sane way to move forward.
> 
> Personally I think that once there's correctly implemented command-line
> option, the feature flag is somewhat redundant (and we've never had one
> for /dev/tpmrm0). And it will help a lot with kernel QA as you can run
> tests with same kernel image without recompilation.

I don't really see any possible security issues either with null seed.

It's no different as per remote attestation when compared storage keys.
In a power cycle it's like same as per TPM2_Certify is considered. It's
pretty much exactly performance issues but depending on deployment.
Sometimes storage key root would be probably a better choice.

I really tried to dig something else than exactly perf stuff but was
unsuccessful to find anything, and I've actually done a lot of work
at work on remote attestation so everything is also somewhat fresh
on my mind.

Still rooting to perf, immediate action being default option disable,
and long term action being replacing the compilation option with
kernel command-line options. I.e., I'll stay on track what I'e
been already doing for some time :-)

That said, my PR cover letter (or the tag message) did suck and
I'll just address next during exactly why something is or isn't
an issue. I think this is really good outcome for everyone in
the long run (because everyone will get the outcome they were
looking for).

BR, Jarkko

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