linux-kernel - Re: [RFC PATCH 2/2] KEYS: implement derived keys

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Date: Wed, 15 May 2024 07:44:07 +0100
From: Ignat Korchagin <ignat@...udflare.com>
To: Jarkko Sakkinen <jarkko@...nel.org>
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 2/2] KEYS: implement derived keys

On Wed, May 15, 2024 at 12:44 AM Jarkko Sakkinen <jarkko@...nel.org> wrote:
>
> On Wed May 15, 2024 at 2:10 AM EEST, Jarkko Sakkinen wrote:
> > On Sat May 4, 2024 at 1:16 AM EEST, Ignat Korchagin wrote:
> > > Derived keys are similar to user keys, but their payload is derived from the
> > > primary TPM seed and some metadata of the requesting process. This way every
> >
> > What is exactly "some metadata"?
> >
> > > application can get a unique secret/key, which is cryptographically bound to
> >
> > What is "cryptographically bound". Please go straight to the point and
> > cut out *all* white paper'ish phrases. We do not need it and will make
> > painful to backtrack this commit once in the mainline.
> >
> > > the TPM without the need to provide the key material externally (unlike trusted
> > > keys). Also, the whole key derivation process is deterministic, so as long as
> >
> > Why trusted keys is inside braces. It is not important for the point
> > you are trying to make here?
> >
> > > the TPM is available, applications can always recover their keys, which may
> > > allow for easier key management on stateless systems.
> >
> > Please drop "stateless system" unless you provide a rigid definition
> > what it is. I have no idea what you mean by it. Probably not that
> > important, right?
> >
> > >
> > > In this implementation the following factors will be used as a key derivation
> > > factor:
> > >   * requested key length
> > >   * requesting process effective user id
> > >   * either the application executable path or the application integrity
> > >     metadata (if available)
> >
> > NAK for path for any possible key derivation. They are racy and
> > and ambiguous.
> >
> > This should have been in the beginning instead of "some data". What
> > other implementations exist. For me "this implementation" implies
> > that this one competing alternative to multiple implementations
> > of the same thing.
> >
> > I do not like this science/white paper style at all. Just express
> > short, open code everything right at start when you need and cut
> > extras like "stateless system" unless you can provide exact, sound
> > and unambiguous definiton of it.
> >
> > Just want to underline how this really needs a complete rewrite with
> > clear and concise explanation :-) This won't ever work.
> >
> > >
> > > Key length is used so requests for keys with different sizes result in keys
> > > with different cryptographic material.
> >
> > What is "key length"? Please refer the exact attribute.
> >
> > >
> > > User id is mixed, so different users get different keys even when executing the
> >
> > First of all it would be more clear to just s/User id/UID/
> >
> > And make obvious whether we are talking about ruid or euid and how
> > this interacts with GIDs.
> >
> > I'll look at the code change next round if the commit message starts
> > making any sense.
>
> Right and neither UIDs and GIDs are applicable for key derivation for
> quite obvious reasons. So NAK for that too.

Can you, please, clarify a bit here? Not very obvious for me. I added
euid for two reasons:
  * an unprivileged user might run a normally privileged application,
for example /usr/sbin/sshd, and depending on the code could "leak" the
key
  * without it and with unprivileged user namespaces it is possible to
create an unprivileged container with code at the same path as a
privileged application

Why do you think UIDs/GIDs are not applicable as mixins?

Ignat

> You can make them point out unlimited different identities...
>
> BR, Jarkko