When the flash is not owned by the non-secure world, accessing the EFI
variables is straight-forward and done via EFI Runtime Variable
Services.  In this case, critical variables for system integrity and
security are normally stored in the dedicated secure storage and can
only be manipulated directly from the secure world.

Usually, small embedded devices don't have the special dedicated secure
storage. The eMMC device with an RPMB partition is becoming more common,
and we can use this RPMB partition to store the EFI Variables.

The eMMC device is typically owned by the non-secure world (Linux in our
case). There is an existing solution utilizing eMMC RPMB partition for
EFI Variables, it is implemented by interacting with TEE (OP-TEE in this
case), 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 TEE and StandaloneMM.

So let's add the kernel functions needed.

This feature is implemented as a kernel module.  StMM PTA has
TA_FLAG_DEVICE_ENUM_SUPP flag when registered to OP-TEE so that this
tee_stmm_efi module is probed after tee-supplicant starts, since
"SetVariable" EFI Runtime Variable Service requires to interact with
tee-supplicant.
Acked-by: Sumit Garg <sumit.garg@linaro.org>
Co-developed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>