linux-kernel - Re: Migration to trusted keys: sealing user-provided key?

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Date:   Sun, 31 Jan 2021 07:09:55 -0500
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Jarkko Sakkinen <jarkko@...nel.org>,
        Ahmad Fatoum <a.fatoum@...gutronix.de>,
        James Bottomley <jejb@...ux.ibm.com>,
        David Howells <dhowells@...hat.com>, keyrings@...r.kernel.org
Cc:     linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org, kernel@...gutronix.de,
        jlu@...gutronix.de
Subject: Re: Migration to trusted keys: sealing user-provided key?

On Sat, 2021-01-30 at 19:53 +0200, Jarkko Sakkinen wrote:
> On Thu, 2021-01-28 at 18:31 +0100, Ahmad Fatoum wrote:
> > Hello,
> > 
> > I've been looking into how a migration to using trusted/encrypted keys
> > would look like (particularly with dm-crypt).
> > 
> > Currently, it seems the the only way is to re-encrypt the partitions
> > because trusted/encrypted keys always generate their payloads from
> > RNG.
> > 
> > If instead there was a key command to initialize a new trusted/encrypted
> > key with a user provided value, users could use whatever mechanism they
> > used beforehand to get a plaintext key and use that to initialize a new
> > trusted/encrypted key. From there on, the key will be like any other
> > trusted/encrypted key and not be disclosed again to userspace.
> > 
> > What are your thoughts on this? Would an API like
> > 
> >   keyctl add trusted dmcrypt-key 'set <content>' # user-supplied content
> > 
> > be acceptable?
> 
> Maybe it's the lack of knowledge with dm-crypt, but why this would be
> useful? Just want to understand the bottleneck, that's all.

We upstreamed "trusted" & "encrypted" keys together in order to address
this sort of problem.   Instead of directly using a "trusted" key for
persistent file signatures being stored as xattrs, the "encrypted" key
provides one level of indirection.   The "encrypted" key may be
encrypted/decrypted with either a TPM based "trusted" key or with a
"user" type symmetric key[1].

Instead of modifying "trusted" keys, use a "user" type "encrypted" key.

Mimi

[1] The ima-evm-utils README contains EVM examples of "trusted" and
"user" based "encrypted" keys.