On Tue, 6 Jun 2023 at 12:28, Ard Biesheuvel <ardb@xxxxxxxxxx> wrote: > > On Tue, 6 Jun 2023 at 08:52, Sumit Garg <sumit.garg@xxxxxxxxxx> wrote: > > > > Hi Jan, > > > > On Tue, 6 Jun 2023 at 12:09, Jan Kiszka <jan.kiszka@xxxxxxxxxxx> wrote: > > > > > > On 26.05.23 03:07, Masahisa Kojima wrote: > > > > When the flash is not owned by the non-secure world, accessing the EFI > > > > variables is straightforward 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 only accessible > > > > from the secure world. > > > > > > > > On the other hand, the small embedded devices don't have the special > > > > dedicated secure storage. The eMMC device with an RPMB partition is > > > > becoming more common, we can use an RPMB partition to store the > > > > EFI Variables. > > > > > > > > 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 > > > > 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@xxxxxxxxxx> > > > > Co-developed-by: Ilias Apalodimas <ilias.apalodimas@xxxxxxxxxx> > > > > Signed-off-by: Ilias Apalodimas <ilias.apalodimas@xxxxxxxxxx> > > > > Signed-off-by: Masahisa Kojima <masahisa.kojima@xxxxxxxxxx> > > > > --- > > > > drivers/firmware/efi/Kconfig | 15 + > > > > drivers/firmware/efi/Makefile | 1 + > > > > drivers/firmware/efi/stmm/mm_communication.h | 236 +++++++ > > > > drivers/firmware/efi/stmm/tee_stmm_efi.c | 638 +++++++++++++++++++ > > > > 4 files changed, 890 insertions(+) > > > > create mode 100644 drivers/firmware/efi/stmm/mm_communication.h > > > > create mode 100644 drivers/firmware/efi/stmm/tee_stmm_efi.c > > > > > ... > > > > > > I think we have a probe ordering issue with this driver: > > > efivarfs_fill_super() may be called before the TEE bus was probed, thus > > > with the default efivar ops still registered. And that means > > > efivar_supports_writes() will return false, and the fs declares itself > > > as readonly. I've seen systemd mounting it r/o initialling, and you need > > > to remount the fs to enable writability. > > > > > > Is there anything that could be done to re-order things reliably, probe > > > the tee bus earlier etc.? > > > > This driver has a dependency on user-space daemon: tee-supplicant to > > be running for RPMB access. So once you start that daemon the > > corresponding device will be enumerated on the TEE bus and this driver > > probe will be invoked. So I would suggest you to load this daemon very > > early in the boot process or better to make it a part of initramfs. > > > > That is not the point, really. > > If this dependency exists, the code should be aware of that, and made > to work correctly in spite of it. Requiring a module to be part of > initramfs is not a reasonable fix. I am not sure if I followed you here. Until we have the tee-stmm-efi module loaded we won't get corresponding EFI operations registered. The key here is that the underlying OP-TEE device won't be available until tee-supplicant (see enumeration with PTA_CMD_GET_DEVICES_SUPP) is running. Do you have any better ideas regarding how this should be handled? > > IIUC, this also means that the efivar ops are updated while there is > already a client. This seems less than ideal as well That's true. An ideal situation would be to allow in-kernel RPMB access APIs for OP-TEE or other kernel drivers to use. With that we should be able to remove this tee-supplicant dependency. I hope you are already aware about the earlier efforts to add the RPMB subsystem to the kernel. But with these real world use-cases emerging like EFI variables and fTPM, we should be able to convince corresponding subsystem maintainers. -Sumit