Re: [PATCH v5 0/4] Introduce TEE based Trusted Keys support

From: Sumit Garg
Date: Wed Jun 03 2020 - 04:08:03 EST

On Tue, 2 Jun 2020 at 20:14, James Bottomley <jejb@xxxxxxxxxxxxx> wrote:
> On Tue, 2020-06-02 at 19:48 +0530, Sumit Garg wrote:
> > Add support for TEE based trusted keys where TEE provides the
> > functionality to seal and unseal trusted keys using hardware unique
> > key. Also, this is an alternative in case platform doesn't possess a
> > TPM device.
> So here's a meta problem: in the case when the platform possesses both
> TEE and TPM what should it do?

IMO, trust source (either a TPM or a TEE) should be unique and
carefully chosen as per platform security policy corresponding to a
particular threat model.

And moreover TEEs have been mostly used in the embedded world where
having a hardware TPM is cumbersome given constraints regarding BoM
cost and hardware resources.

> Things like this:
> > --- a/security/keys/trusted-keys/trusted_core.c
> > +++ b/security/keys/trusted-keys/trusted_core.c
> > @@ -25,6 +25,8 @@
> >
> > #if defined(CONFIG_TRUSTED_TPM)
> > static struct trusted_key_ops *trusted_key_ops =
> > &tpm_trusted_key_ops;
> > +#elif defined(CONFIG_TRUSTED_TEE)
> > +static struct trusted_key_ops *trusted_key_ops =
> > &tee_trusted_key_ops;
> > #else
> Say it's either/or at a Kconfig level: so if you select both TEE and
> TPM based trusted keys at compile time, we intall the TPM ops and
> ignore the TEE ops, is that right? Surely this should be runtime
> selectable based on what the platform has ...

This dynamic selection was already part of v4 patch-set but after
objection from Jarrko here [1], I switched to compile time mode


> perhaps it should even be
> selectable per key?
> Once it is runtime selectable, what should be selected in the both
> case? Or should we allow the user to decide, if so, how?
> when you pipe a trusted key, I think the subtype (TEE or TPM) should be
> part of the piped information, so it loads again seamlessly. This
> would actually be fixed by something like the ASN.1 scheme I'm trying
> to upstream, at least for TPM keys, but do TEE keys have a recognized
> ASN.1 format?

I guess this is something which we can refine later if there are real
platforms that have a particular security requirement to support both
TPM and a TEE.


> James