linux-kernel - Re: [RFC PATCH 0/2] TPM derived keys

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Date: Tue, 14 May 2024 18:42:55 +0300
From: "Jarkko Sakkinen" <jarkko@...nel.org>
To: "Ignat Korchagin" <ignat@...udflare.com>
Cc: "James Bottomley" <James.Bottomley@...senpartnership.com>, "Mimi Zohar"
 <zohar@...ux.ibm.com>, "David Howells" <dhowells@...hat.com>, "Paul Moore"
 <paul@...l-moore.com>, "James Morris" <jmorris@...ei.org>,
 <serge@...lyn.com>, <linux-integrity@...r.kernel.org>,
 <keyrings@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
 <kernel-team@...udflare.com>
Subject: Re: [RFC PATCH 0/2] TPM derived keys

On Tue May 14, 2024 at 6:30 PM EEST, Ignat Korchagin wrote:
> On Tue, May 14, 2024 at 4:26 PM Jarkko Sakkinen <jarkko@...nel.org> wrote:
> >
> > On Tue May 14, 2024 at 6:21 PM EEST, Jarkko Sakkinen wrote:
> > > On Tue May 14, 2024 at 5:30 PM EEST, Jarkko Sakkinen wrote:
> > > > On Tue May 14, 2024 at 5:00 PM EEST, Jarkko Sakkinen wrote:
> > > > > On Tue May 14, 2024 at 4:11 PM EEST, Ignat Korchagin wrote:
> > > > > > For example, a cheap NAS box with no internal storage (disks connected
> > > > > > externally via USB). We want:
> > > > > >   * disks to be encrypted and decryptable only by this NAS box
> > > > >
> > > > > So how this differs from LUKS2 style, which also systemd supports where
> > > > > the encryption key is anchored to PCR's? If I took hard drive out of my
> > > > > Linux box, I could not decrypt it in another machine because of this.
> > > >
> > > > Maybe you could replace the real LUKS2 header with a dummy LUKS2
> > > > header, which would need to be able the describe "do not use this" and
> > > > e.g. SHA256 of the actual header. And then treat the looked up header as
> > > > the header when the drive is mounted.
> > > >
> > > > LUKS2 would also need to be able to have pre-defined (e.g. kernel
> > > > command-line or bootconfig) small internal storage, which would be
> > > > also encrypted with TPM's PRCs containing an array of LUKS2 header
> > > > and then look up that with SHA256 as the key.
> > > >
> > > > Without knowing LUKS2 implementation to me these do not sound reaching
> > > > the impossible engineer problems so maybe this would be worth of
> > > > investigating...
> > >
> > > Or why you could not just encrypt the whole header with another key
> > > that is only in that device? Then it would appear as random full
> > > length.
> > >
> > > I.e. unsealing
> > >
> > > 1. Decrypt LUKS2 header with TPM2 key
> > > 2. Use the new resulting header as it was in the place of encrypted
> > >    stored to the external drive.
> > > 3. Decrypt key from the LUK2S header etc.
> >
> > Maybe something like:
> >
> > 1. Asymmetric for LUKS2 (just like it is)
> > 2. Additional symmetric key, which is created as non-migratable and stored
> >    to the TPM2 chip. This deciphers the header, i.e. takes the random
> >    away.
>
> This could work, but you still have the problem of - if the header
> gets wiped, all the data is lost.
> As for storing things on the TPM chip - that doesn't scale. Today you
> only think about disk encryption, tomorrow there is a new application,
> which wants to do the same thing and so on. One of the features of
> derived keys - you don't store anything, just recreate/derive when
> needed and it scales infinitely.

OK, so now I know the problem at least and that is probably the
most important thing in this discussion, right?

So make a better story, now you also probably have better idea,
also split the patch properly by subsystem, send the patch set,
and I'll promise to revisit.

Fair enough? :-)

BR, Jarkko