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

Ignat Korchagin
2024-05-14T16:09:08Z

On Tue, May 14, 2024 at 4:43 PM 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 PM 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 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?

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