Re: [PATCH v2 1/3] rpmb: add Replay Protected Memory Block (RPMB) subsystem

[Sumit Garg]

On Wed, 7 Feb 2024 at 12:56, Jens Wiklander <jens.wiklander@xxxxxxxxxx> wrote:
>
> H,
>
> On Wed, Feb 7, 2024 at 7:11 AM Sumit Garg <sumit.garg@xxxxxxxxxx> wrote:
> >
> > Hi Ilias, Ulf,
> >
> > On Tue, 6 Feb 2024 at 20:41, Ilias Apalodimas
> > <ilias.apalodimas@xxxxxxxxxx> wrote:
> > >
> > > Hi Ulf,
> > >
> > > On Tue, 6 Feb 2024 at 14:34, Ulf Hansson <ulf.hansson@xxxxxxxxxx> wrote:
> > > >
> > > > On Wed, 31 Jan 2024 at 18:44, Jens Wiklander <jens.wiklander@linaroorg> wrote:
> > > > >
> > > > > A number of storage technologies support a specialised hardware
> > > > > partition designed to be resistant to replay attacks. The underlying
> > > > > HW protocols differ but the operations are common. The RPMB partition
> > > > > cannot be accessed via standard block layer, but by a set of specific
> > > > > RPMB commands: WRITE, READ, GET_WRITE_COUNTER, and PROGRAM_KEY. Such a
> > > > > partition provides authenticated and replay protected access, hence
> > > > > suitable as a secure storage.
> > > > >
> > > > > The initial aim of this patch is to provide a simple RPMB Driver which
> > > > > can be accessed by the optee driver to facilitate early RPMB access to
> > > > > OP-TEE OS (secure OS) during the boot time.
> > > >
> > > > How early do we expect OP-TEE to need RPMB access?
> > >
> > > It depends on the requested services. I am currently aware of 2
> > > services that depend on the RPMB
> > > - FirmwareTPM
> > > - UEFI variables stored there via optee.
> > >
> > > For the FirmwareTPM it depends on when you want to use it. This
> > > typically happens when the initramfs is loaded or systemd requests
> > > access to the TPM. I guess this is late enough to not cause problems?
> >
> > Actually RPMB access is done as early as during fTPM probe, probably
> > to cache NVRAM from RPMB during fTPM init. Also, there is a kernel
> > user being IMA which would require fTPM access too. So we really need
> > to manage dependencies here.
> >
> > >
> > > For the latter, we won't need the supplicant until a write is
> > > requested. This will only happen once the userspace is up and running.
> > > The UEFI driver that sits behind OP-TEE has an in-memory cache of the
> > > variables, so all the reads (the kernel invokes get_next_variable
> > > during boot) are working without it.
> > >
> > > Thanks
> > > /Ilias
> > > >
> > > > The way things work for mmc today, is that the eMMC card gets
> > > > discovered/probed via a workqueue. The work is punted by the mmc host
> > > > driver (typically a module-platform-driver), when it has probed
> > > > successfully.
> >
> > It would be nice if RPMB is available as early as possible but for the
> > time being we can try to see if probe deferral suffices for all
> > use-cases.
> >
> > > >
> > > > The point is, it looks like we need some kind of probe deferral
> > > > mechanism too. Whether we want the OP-TEE driver to manage this itself
> > > > or whether we should let rpmb_dev_find_device() deal with it, I don't
> > > > know.
> >
> > I wouldn't like to see the OP-TEE driver probe being deferred due to
> > this since there are other kernel drivers like OP-TEE RNG (should be
> > available as early as we can) etc. which don't have any dependency on
> > RPMB.
>
> I agree, the optee driver itself can probe without RPMB.
>
> >
> > How about for the time being we defer fTPM probe until RPMB is available?
>
> Sounds a bit like what we do with the
> optee_enumerate_devices(PTA_CMD_GET_DEVICES_SUPP) call when
> tee-supplicant has opened the supplicant device. It would perhaps work
> with a PTA_CMD_GET_DEVICES_RPMB or such.

That sounds much better, it will be like an OP-TEE driver callback
(optee_enumerate_devices(PTA_CMD_GET_DEVICES_RPMB)) registered with
the RPMB subsystem. But we should check if all the RPMB partitions are
registered before we invoke the callbacks such that OP-TEE will have a
chance to select the right one.

-Sumit

>
> Thanks,
> Jens