Migration to trusted keys: sealing user-provided key?

Jan Lübbe jlu at pengutronix.de
Mon Feb 1 15:50:58 UTC 2021

On Mon, 2021-02-01 at 11:36 +0000, David Howells wrote:
> Jan Lübbe <jlu at pengutronix.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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