linux-kernel - Re: [RFC 0/7] Introduce TEE based Trusted Keys support

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Message-ID: <CAFA6WYOZOrv5D6261z-bwCP6+6ORzH3PcZz89Sfn=yts0dkvTg@mail.gmail.com>
Date:   Fri, 14 Jun 2019 13:47:16 +0530
From:   Sumit Garg <sumit.garg@...aro.org>
To:     Mimi Zohar <zohar@...ux.ibm.com>
Cc:     Casey Schaufler <casey@...aufler-ca.com>, keyrings@...r.kernel.org,
        linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        Jens Wiklander <jens.wiklander@...aro.org>, corbet@....net,
        dhowells@...hat.com, jejb@...ux.ibm.com,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        jmorris@...ei.org, serge@...lyn.com,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        linux-doc@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        tee-dev@...ts.linaro.org
Subject: Re: [RFC 0/7] Introduce TEE based Trusted Keys support

Thanks Mimi for your comments.

On Fri, 14 Jun 2019 at 05:33, Mimi Zohar <zohar@...ux.ibm.com> wrote:
>
> On Thu, 2019-06-13 at 09:40 -0700, Casey Schaufler wrote:
> > On 6/13/2019 3:30 AM, 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.
> > >
> > > This series also adds some TEE features like:
> >
> > Please expand the acronym TEE on first use. That will
> > help people who don't work with it on a daily basis
> > understand what you're going on about.
>
> Thanks, Casey.
>
> "[6/7] doc: keys: Document usage of TEE based Trusted Keys" refers to
> the kernel tee documentation, but that documentation is limited to
> userspace interaction with the tee.
>

Thanks for pointing this out. I will update documentation to include
TEE bus approach and communication apis for kernel clients.

BTW, the interface is similar as with user-space. Only difference is
instead of IOCTL's from user-space, there are wrapper apis to
communicate with TEE.

Also, in case someone is interested to learn about TEE technology,
this webinar [1] could be one of starting points.

> A trusted key is a random number generated and sealed(encrypted) by
> the TPM, so that only the TPM may unseal it.  The sealing key never
> leaves the TPM.  The sealed, trusted key may be exported to userspace.

Understood.

>  In the tee case, can the "sealing" key ever leave the tee?

No, the "sealing" key never leaves TEE. Its basically a Hardware
Unique Key (HUK) tied to a particular SoC.

>  Can the
> sealed, trusted key, exported to userspace, be unsealed by the tee?

You mean using user-space interface to TEE? If yes, then answer is
"no" as user-space can't communicate with this TEE service as its
accessible to kernel clients only (see patch [2]).

In case you meant loading exported trusted key blob via "keyctl", then
"yes" this driver can unseal the trusted key. Have a look at examples
I have listed in documentation patch [3]. Also, this approach works
well across power cycles too.

>  Are the tee security protections similar to those of the TPM?  How do
> they compare?
>

Let me try to compare both environments. Regarding TEE, I will refer
to OP-TEE [4] as one of its implementation.

TPM:

1. External hardware.
2. Sealing key resides inside TPM.
3. Communicates via SPI, I2C etc.

OP-TEE:

1. On chip, trusted execution environment enforced via ARM TrustZone.
2. Sealing key is unique to a particular SoC provided by secure fuses,
secure crypto engine etc.
3. Communicates via Secure Monitor Calls (SMCs [5]).

[1] https://globalplatform.org/resource-publication/webinar-an-introduction-to-tee-technology/
[2] [RFC 3/7] tee: add private login method for kernel clients
[3] [RFC 6/7] doc: keys: Document usage of TEE based Trusted Keys
[4] https://optee.readthedocs.io/general/about.html
[5] http://infocenter.arm.com/help/topic/com.arm.doc.den0028b/ARM_DEN0028B_SMC_Calling_Convention.pdf

-Sumit

> Mimi
>
> >
> > >
> > > Patch #1, #2 enables support for registered kernel shared memory with TEE.
> > >
> > > Patch #3 enables support for private kernel login method required for
> > > cases like trusted keys where we don't wan't user-space to directly access
> > > TEE service to retrieve trusted key contents.
> > >
> > > Rest of the patches from #4 to #7 adds support for TEE based trusted keys.
> > >
> > > This patch-set has been tested with OP-TEE based pseudo TA which can be
> > > found here [1].
> > >
> > > Looking forward to your valuable feedback/suggestions.
>