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

[Jens Wiklander]

On Mon, Feb 6, 2023 at 7:44 AM Sumit Garg <sumit.garg@xxxxxxxxxx> wrote:
>
> On Fri, 3 Feb 2023 at 16:25, Jens Wiklander <jens.wiklander@xxxxxxxxxx> wrote:
> >
> > 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@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.
> >
> > 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.
>
> Aren't we already adding PTA_STMM?

Yes, something is need to recieve those messages and forward the MM
stuff to secure user space.

>
> Isn't the StMM specific to Arm as you already mentioned it was
> designed to specifically use raw SMCs? So if in future AMD TEE wants
> to implement EFI services, can we suggest they reuse the MM interface?

I wouldn't suggest anything until I understood that problem better.

>
> I am not sure why we need to redirect EFI variables via MM interface
> communication buffers rather than directly using the TEE shared memory
> approach.

I allways assumed that was done in order to keep the changes in StMM
at a mininum compared to non-TEE configurations.

Cheers,
Jens

>
> Ard,
>
> Since you have better insights into how EFI runtime services have to
> be implemented, can you share your opinion here? It may be something I
> am missing here.
>
> -Sumit
>
> >
> > 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
> > > > > >