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

[Sumit Garg]

Hi Jens,

On Fri, 3 Feb 2023 at 13:59, Jens Wiklander <jens.wiklander@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxx> 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.
>

If you look carefully, we are essentially defining an ABI towards the
secure world. The approach in this patch-set adds the MM interface as
a redundant ABI layer which makes it complex to maintain. Now think
about if every TEE implementation would propose such a complex ABI. It
looks like a maintenance nightmare to me.

The concerns you are highlighting about OP-TEE size, I think those are
implementation details which can be simplified later but once you have
defined an ABI then you are stuck with its maintainability.

-Sumit

> 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
> > >