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

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Date:   Sat, 30 Jan 2021 10:07:49 -0800
From:   James Bottomley <jejb@...ux.ibm.com>
To:     Jarkko Sakkinen <jarkko@...nel.org>,
        Ahmad Fatoum <a.fatoum@...gutronix.de>,
        Mimi Zohar <zohar@...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.

There was a recent patch to dm-crypt to add encrypted key support:
27f5411a718c ("dm crypt: support using encrypted keys").  The
implementation requires the actual disk encryption master key to be in
the payload.  Most people don't want to change that key because it
involves re-encrypting the whole disk (usually what people mean when
they say "key" for dm-crypt is a passphrase that decrypts this master
key from a keyslot in the metadata, which is why you can change your
passphrase without changing the underlying encryption).

However, once we get the trusted key rework upstream, we do have a
solution:  The key format becomes interoperable with the
openssl_tpm2_engine and we can now do seal_tpm2_data on any payload and
the kernel will accept it.

James