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

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Date: Tue, 14 May 2024 18:26:14 +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 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.

BR, Jarkko