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

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Message-ID: <64472434a367060ddce6e03425156b8312a5ad6c.camel@pengutronix.de>
Date:   Mon, 01 Feb 2021 16:31:30 +0100
From:   Jan Lübbe <jlu@...gutronix.de>
To:     Mimi Zohar <zohar@...ux.ibm.com>,
        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,
        Sumit Garg <sumit.garg@...aro.org>
Cc:     linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org, kernel@...gutronix.de
Subject: Re: Migration to trusted keys: sealing user-provided key?

On Sun, 2021-01-31 at 09:29 -0500, Mimi Zohar wrote:
> On Sun, 2021-01-31 at 15:14 +0100, Jan Lübbe wrote:
> > On Sun, 2021-01-31 at 07:09 -0500, Mimi Zohar wrote:
> 
> <snip>
> 
> > > 
> > > [1] The ima-evm-utils README contains EVM examples of "trusted" and
> > > "user" based "encrypted" keys.
> > 
> > I assume you refer to
> > https://sourceforge.net/p/linux-ima/ima-evm-utils/ci/master/tree/README#l143
> > "Generate EVM encrypted keys" and "Generate EVM trusted keys (TPM based)"?
> > 
> > In both cases, the key used by EVM is a *newly generated* random key. The only
> > difference is whether it's encrypted to a user key or a (random) trusted key.
> 
> The "encrypted" asymmetric key data doesn't change, "update" just
> changes the key under which it is encrypted/decrypted.
> 
> Usage::
> 
>     keyctl add encrypted name "new [format] key-type:master-key-name keylen"
>         ring
>     keyctl add encrypted name "load hex_blob" ring

'load' (as I understand the code) only accepts an encrypted blob.

So the only way I see to have an encrypted key with a non-random key data would
be:
- create a random temporary master key and load a copy as a user key
- encrypt the chosen key data with the temporary master key (using a new
userspace reimplementation of the kernel encrypted key blob format)
- use keyctl add encrypted dmcrypt "load <encrypted blob>" <keyring>
- create new trusted master key (OP-TEE or CAAM in our case) as 
- use keyctl update to switch to the new trusted master key
- use keyctl pipe on the trusted and encrypted keys and store both for loading
on later boots

If we'd support importing a pre-existing key into a trusted or encrypted key,
we'd do instead:
- use keyctl add trusted dmcrypt "import <unencrypted key data>"
- use keyctl pipe on the trusted key and store it for loading on later boots

This way, users wouldn't need to care which backend is used by trusted keys
(TPM/OP-TEE/CAAM/...). That would make use-cases where a random key is not
suitable as straight-forward as the those where a random key is OK.

Best regards
Jan

>     keyctl update keyid "update key-type:master-key-name"
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