Received-SPF: pass (google.com: domain of linux-kernel+bounces-178915-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45e3:2400::1 as permitted sender) client-ip=2604:1380:45e3:2400::1;
Precedence: bulk
MIME-Version: 1.0
References: <20240503221634.44274-1-ignat@cloudflare.com> <CALrw=nGhgRrhJ5mWWC6sV2WYWoijvD9WgFzMfOe6mHmqnza-Hw@mail.gmail.com>
 <D18XXJ373C2V.2M6AOMKD1B89W@kernel.org> <CALrw=nHGLN=dn3fbyAcXsBufw0tAWUT1PKVHDK5RZkHcdd3CUw@mail.gmail.com>
 <D19CUF0H9Q3S.3L5Y5S9553S5@kernel.org> <CALrw=nEZ07U9VhbGsnpchOYw1icUZCnuoHHXkJLzhFqSPe9_fQ@mail.gmail.com>
 <D19F74M6B8UC.2VEOOZHGOS87V@kernel.org> <D19FUGDA2CUO.16EF7U9ZEZ4SD@kernel.org>
 <D19GWXHYP2VC.1OY7BOW5LNXVF@kernel.org> <D19H0UVF3R0O.3N4GLZWFRZ2DO@kernel.org>
 <CALrw=nE7ga6wxSqrJBTOaj+pPXhi4+-Rn4ePRC9vXL-8Qd3GrA@mail.gmail.com> <D19HDMZ1OKN9.2PX2FJVY4WZ09@kernel.org>
In-Reply-To: <D19HDMZ1OKN9.2PX2FJVY4WZ09@kernel.org>
From: Ignat Korchagin <ignat@cloudflare.com>
Date: Tue, 14 May 2024 17:08:56 +0100
Message-ID: <CALrw=nF53qsmBp06qDerh3++0VYm-OZJ8k-Es3+2AAQFRjY-9w@mail.gmail.com>
Subject: Re: [RFC PATCH 0/2] TPM derived keys
To: Jarkko Sakkinen <jarkko@kernel.org>
Cc: James Bottomley <James.Bottomley@hansenpartnership.com>, Mimi Zohar <zohar@linux.ibm.com>, 
	David Howells <dhowells@redhat.com>, Paul Moore <paul@paul-moore.com>, 
	James Morris <jmorris@namei.org>, serge@hallyn.com, linux-integrity@vger.kernel.org, 
	keyrings@vger.kernel.org, linux-kernel@vger.kernel.org, 
	kernel-team@cloudflare.com
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

On Tue, May 14, 2024 at 4:43=E2=80=AFPM Jarkko Sakkinen <jarkko@kernel.org>=
 wrote:
>
> On Tue May 14, 2024 at 6:30 PM EEST, Ignat Korchagin wrote:
> > On Tue, May 14, 2024 at 4:26=E2=80=AFPM Jarkko Sakkinen <jarkko@kernel.=
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 bo=
x
> > > > > >
> > > > > > So how this differs from LUKS2 style, which also systemd suppor=
ts where
> > > > > > the encryption key is anchored to PCR's? If I took hard drive o=
ut 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 he=
ader 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 b=
e
> > > > > also encrypted with TPM's PRCs containing an array of LUKS2 heade=
r
> > > > > and then look up that with SHA256 as the key.
> > > > >
> > > > > Without knowing LUKS2 implementation to me these do not sound rea=
ching
> > > > > 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 s=
tored
> > >    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?

Yes, I think so.

> So make a better story, now you also probably have better idea,
> also split the patch properly by subsystem, send the patch set,

I'm actually not super clear on this part - I have two patches: one
for TPM header definitions and another one for the keyring subsystem?
Any other subsystems in play here?

> and I'll promise to revisit.

Thanks. Would probably take some time as I want to think more on the
open questions I raised in the description, try to address some
comments from James B from other replies (key rotation for example)
and rebase on recently merged TPM encrypted sessions. But since this
is an RFC I would like to continue the discussion and gather opinions
from folks here, if there are any more concerns.

> Fair enough? :-)
>
> BR, Jarkko