From: Ignat Korchagin <ignat@cloudflare.com>
To: James Bottomley <James.Bottomley@hansenpartnership.com>
Cc: Jarkko Sakkinen <jarkko@kernel.org>,
Ben Boeckel <me@benboeckel.net>,
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
Subject: Re: [RFC PATCH 0/2] TPM derived keys
Date: Tue, 14 May 2024 10:50:51 +0100 [thread overview]
Message-ID: <CALrw=nFOh0=TXGx-z_oTkLWshVU_AfGRQzcC3zxVTzcRbuRqQQ@mail.gmail.com> (raw)
In-Reply-To: <44cd50b60a0a4e376d01544d25187556b8badf94.camel@HansenPartnership.com>
On Mon, May 13, 2024 at 11:33 PM James Bottomley
<James.Bottomley@hansenpartnership.com> wrote:
>
> On Mon, 2024-05-13 at 18:09 +0100, Ignat Korchagin wrote:
> [...]
> > TPM derived keys attempt to address the above use cases by allowing
> > applications to deterministically derive unique cryptographic keys
> > for their own purposes directly from the TPM seed in the owner
> > hierarchy. The idea is that when an application requests a new key,
> > instead of generating a random key and wrapping it with the TPM, the
> > implementation generates a key via KDF(hierarchy seed, application
> > specific info). Therefore, the resulting keys will always be
> > cryptographically bound to the application itself and the device they
> > were generated on.
>
> So I think what confuses me is what the expected cryptographic secrecy
> properties of the derived keys are. I get they're a KDF of seed and
> deterministic properties, but if those mixing values are well known (as
> the path or binary checksum cases) then anyone with access to the TPM
> can derive the key from user space because they can easily obtain the
> mixing parameters and there's no protection to the TPM keyed hash
> operation.
>
> Consider the use case where two users are using derived keys on the
> same system (so same TPM). Assuming they use them to protect sensitive
> information, what prevents user1 from simply deriving user2's key and
> getting the information, or am I missing the point of this?
You are correct: it is possible, but in practice it would be limited
only to privileged users/applications. I remember there was a push to
set a 666 mask for the TPM device file, but it is not how it is done
today by default. Also I think the same applies to trusted keys as
well, at least without any additional authorizations or PCR
restrictions on the blob (I remember I could manually unwrap a trusted
key blob in userspace as root).
It would be fixed if we could limit access to some TPM ops only from
the kernel, but I remember from one of your presentations that it is
generally a hard problem and that some solution was in the works (was
it based on limiting access to a resettable PCR?). I'm happy to
consider adopting it here as well somehow.
> James
>
next prev parent reply other threads:[~2024-05-14 9:51 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-05-03 22:16 [RFC PATCH 0/2] TPM derived keys Ignat Korchagin
2024-05-03 22:16 ` [RFC PATCH 1/2] tpm: add some algorithm and constant definitions from the TPM spec Ignat Korchagin
2024-05-14 22:51 ` Jarkko Sakkinen
2024-05-14 22:52 ` Jarkko Sakkinen
2024-05-03 22:16 ` [RFC PATCH 2/2] KEYS: implement derived keys Ignat Korchagin
2024-05-14 23:10 ` Jarkko Sakkinen
2024-05-14 23:44 ` Jarkko Sakkinen
2024-05-15 0:00 ` Jarkko Sakkinen
2024-05-15 6:44 ` Ignat Korchagin
2024-05-15 12:00 ` Jarkko Sakkinen
2024-05-15 12:03 ` Jarkko Sakkinen
2024-05-15 7:26 ` Ignat Korchagin
2024-05-04 0:21 ` [RFC PATCH 0/2] TPM " Jarkko Sakkinen
2024-05-04 13:55 ` Ben Boeckel
2024-05-04 14:51 ` Jarkko Sakkinen
2024-05-04 15:35 ` Jarkko Sakkinen
2024-05-13 17:09 ` Ignat Korchagin
2024-05-13 22:33 ` James Bottomley
2024-05-14 9:50 ` Ignat Korchagin [this message]
2024-05-14 14:11 ` James Bottomley
2024-05-14 14:54 ` Ignat Korchagin
2024-05-13 17:11 ` Ignat Korchagin
2024-05-14 0:28 ` Jarkko Sakkinen
2024-05-14 10:05 ` Ignat Korchagin
2024-05-14 12:09 ` Jarkko Sakkinen
2024-05-14 13:11 ` Ignat Korchagin
2024-05-14 14:00 ` Jarkko Sakkinen
2024-05-14 14:30 ` Jarkko Sakkinen
2024-05-14 15:21 ` Jarkko Sakkinen
2024-05-14 15:26 ` Jarkko Sakkinen
2024-05-14 15:30 ` Ignat Korchagin
2024-05-14 15:42 ` Jarkko Sakkinen
2024-05-14 16:08 ` Ignat Korchagin
2024-05-14 16:22 ` Jarkko Sakkinen
2024-05-14 14:41 ` Ignat Korchagin
2024-05-14 14:45 ` Jarkko Sakkinen
2024-05-14 15:30 ` James Bottomley
2024-05-14 15:38 ` Ignat Korchagin
2024-05-14 15:54 ` James Bottomley
2024-05-14 16:01 ` Ignat Korchagin
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