linux-kernel - Re: [RFC PATCH 0/2] introduce op-tee based EFI Runtime Variable Service

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Date:   Fri, 3 Feb 2023 09:29:01 +0100
From:   Jens Wiklander <jens.wiklander@...aro.org>
To:     Sumit Garg <sumit.garg@...aro.org>
Cc:     Masahisa Kojima <masahisa.kojima@...aro.org>,
        Ard Biesheuvel <ardb@...nel.org>, linux-kernel@...r.kernel.org,
        op-tee@...ts.trustedfirmware.org,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Johan Hovold <johan+linaro@...nel.org>
Subject: Re: [RFC PATCH 0/2] introduce op-tee based EFI Runtime Variable
 Service

Hi Sumit,

On Thu, Feb 02, 2023 at 05:35:49PM +0530, Sumit Garg wrote:
> Hi Masahisa,
> 
> On Thu, 26 Jan 2023 at 18:52, Masahisa Kojima
> <masahisa.kojima@...aro.org> wrote:
> >
> > This RFC series introduces the op-tee based EFI Runtime Variable
> > Service.
> >
> > The eMMC device is typically owned by the non-secure world(linux in
> > this case). There is an existing solution utilizing eMMC RPMB partition
> > for EFI Variables, it is implemented by interacting with
> > OP-TEE, StandaloneMM(as EFI Variable Service Pseudo TA), eMMC driver
> > and tee-supplicant. The last piece is the tee-based variable access
> > driver to interact with OP-TEE and StandaloneMM.
> >
> 
> After an overall look at the APIs, following are some initial comments:
> - Is there any reason to have the edk2 specific StandaloneMM stack in
> Linux to communicate with OP-TEE pseudo TA?
> - I think the OP-TEE pseudo TA should be able to expose a rather
> generic invoke commands such as:
>      TEE_EFI_GET_VARIABLE
>      TEE_EFI_GET_NEXT_VARIABLE
>      TEE_EFI_SET_VARIABLE
>   So it should no longer be tied to StMM stack and other TEE
> implementations can re-use the abstracted interface to communicate
> with its corresponding secure storage TA.

In the current setup we have the following layers in the kernel:
1. efivar_operations
2. MM
3. PTA_STMM
4. OP-TEE MSG

and in the secure world:
S1. internal to StMM
S2. MM interface to StMM
S3. PTA_STMM
S4. OP-TEE MSG

If I understand you correctly you'd like to see this instead:
Kernel:
1. efivar_operations
2. PTA_EFIVAR
4. OP-TEE MSG

Since we still have the MM interface with StMM we'd have this in the secure
world:
S1. internal to StMM
S2. MM interface to StMM
S3. PTA_EFIVAR
S4. OP-TEE MSG

At S3 we'd have to convert between EFIVAR and MM messages. The
difference is that we're moving the EFIVAR <-> MM conversion from the
non-secure world into the secure world. We're still using OP-TEE
specific communication at the fourth layer. So we're only moving problem
around, I'd rather avoid growing the OP-TEE part in the secure world.

Cheers,
Jens

> 
> -Sumit
> 
> > Masahisa Kojima (2):
> >   efi: expose efivar generic ops register function
> >   tee: Add op-tee helper functions for variable access
> >
> >  drivers/firmware/efi/efi.c           |  12 +
> >  drivers/tee/optee/Kconfig            |  10 +
> >  drivers/tee/optee/Makefile           |   1 +
> >  drivers/tee/optee/mm_communication.h | 249 +++++++++++
> >  drivers/tee/optee/optee_private.h    |   5 +-
> >  drivers/tee/optee/optee_stmm_efi.c   | 598 +++++++++++++++++++++++++++
> >  drivers/tee/tee_core.c               |  23 ++
> >  include/linux/efi.h                  |   4 +
> >  include/linux/tee_drv.h              |  23 ++
> >  9 files changed, 924 insertions(+), 1 deletion(-)
> >  create mode 100644 drivers/tee/optee/mm_communication.h
> >  create mode 100644 drivers/tee/optee/optee_stmm_efi.c
> >
> > --
> > 2.30.2
> >