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

[Jens Wiklander <jens.wiklander@linaro.org>]

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On Fri, Feb 03, 2023 at 03:03:34PM +0530, Sumit Garg wrote:
> Hi Jens,
> 
> On Fri, 3 Feb 2023 at 13:59, Jens Wiklander <jens.wiklander@linaro.org> 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@linaro.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.
> >
> 
> 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.

You have a point, but keep in mind that it's StMM that matters here.
StMM uses the MM protocol. It was originially using raw SMCs as a
conduit, but with the need for OP-TEE accessing RPMB that's not usable.
So instead we use OP-TEE MSG as a conduit. Seen from that perspective
we're only resuing something established instead of inventing something
new.

Cheers,
Jens

> 
> -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
> > > >