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

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Date: Tue, 14 May 2024 18:21:06 +0300
From: "Jarkko Sakkinen" <jarkko@...nel.org>
To: "Jarkko Sakkinen" <jarkko@...nel.org>, "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 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.

?

BR, Jarkko