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

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Message-ID: <6b362abd95b116e26c65809a3a1525c7951ed0bd.camel@pengutronix.de>
Date:   Mon, 01 Feb 2021 16:50:58 +0100
From:   Jan Lübbe <jlu@...gutronix.de>
To:     David Howells <dhowells@...hat.com>
Cc:     Mimi Zohar <zohar@...ux.ibm.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Ahmad Fatoum <a.fatoum@...gutronix.de>,
        James Bottomley <jejb@...ux.ibm.com>, keyrings@...r.kernel.org,
        Sumit Garg <sumit.garg@...aro.org>,
        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 Mon, 2021-02-01 at 11:36 +0000, David Howells wrote:
> Jan Lübbe <jlu@...gutronix.de> wrote:
> 
> > ... But at this point, you can still do 'keyctl read' on that key, exposing
> > the key material to user space.
> 
> I wonder if it would help to provide a keyctl function to mark a key as being
> permanently unreadable - so that it overrides the READ permission bit.
> 
> Alternatively, you can disable READ and SETATTR permission - but that then
> prevents you from removing other perms if you want to :-/

That would mean using user type keys, right? Then we'd still have the core
problem how a master key can be protected against simply reading it from
flash/disk, as it would be unencrypted in this scenario.

Maybe a bit of background:

We're looking at the trusted/encrypted keys because we want to store the key
material in an encrypted format, only loadable into the same system where they
were generated and only if that's in a trusted state (to solve the master key
problem above).

This binding can be done with trusted keys via a TPM (and soon with Sumit's OP-
TEE backend, or later based on SoC-specific hardware like NXP's CAAM). In the
OP-TEE/CAAM case, the bootloader would ensure that the backend can only be used
when booting a correctly authenticated kernel.

Of course, that's not as flexible as TPMs with a custom policy, but much simpler
and a good fit for many embedded use-cases.

Best regards,
Jan Lübbe
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