Message-ID: <aOUl-mZkRwJJwtJ7@kernel.org>
Date: Tue, 7 Oct 2025 17:38:50 +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 Tue, Oct 07, 2025 at 05:32:24PM +0300, Jarkko Sakkinen wrote:
> 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).

And in the business I'm ATM i.e., covering like with 20-30% market share
Finnish high school IT deplyoment going extremes is feasible, as some
kids are smart and capable of hacking the systems so to speak ;-) I'd
likely enable this feature e.g., in that type of deployment together
with remote attesation just to max out defence in depth.

BR, Jarkko

Powered by blists - more mailing lists