Re: [RFC, PATCH 1/1] rpmb: add Replay Protected Memory Block (RPMB) driver

[Jens Wiklander]

On Mon, Aug 21, 2023 at 12:03 PM Sumit Garg <sumit.garg@xxxxxxxxxx> wrote:
>
> On Mon, 21 Aug 2023 at 15:19, Jerome Forissier
> <jerome.forissier@xxxxxxxxxx> wrote:
> >
> >
> >
> > On 8/17/23 01:31, Shyam Saini wrote:
> > >
> > > Hi Ulf,
> > >
> > >> On Sat, 22 Jul 2023 at 03:41, Shyam Saini
> > >> <shyamsaini@xxxxxxxxxxxxxxxxxxx> wrote:
> > >>>
> > >>> From: Alex Bennée <alex.bennee@xxxxxxxxxx>
> > >>>
> > >>> [This is patch 1 from [1] Alex's submission and this RPMB layer was
> > >>> originally proposed by [2]Thomas Winkler ]
> > >>>
> > >>> 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
> > >>> 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 Linux's optee driver to facilitate fast-path for
> > >>> RPMB access to optee OS(secure OS) during the boot time. [1] Currently,
> > >>> Optee OS relies on user-tee supplicant to access eMMC RPMB partition.
> > >>>
> > >>> A TEE device driver can claim the RPMB interface, for example, via
> > >>> class_interface_register(). The RPMB driver provides a series of
> > >>> operations for interacting with the device.
> > >>
> > >> I don't quite follow this. More exactly, how will the TEE driver know
> > >> what RPMB device it should use?
> > >
> > > I don't have complete code to for this yet, but i think OP-TEE driver
> > > should register with RPMB subsystem and then we can have eMMC/UFS/NVMe
> > > specific implementation for RPMB operations.
> > >
> > > Linux optee driver can handle RPMB frames and pass it to RPMB subsystem
> > >
>
> It would be better to have this OP-TEE use case fully implemented. So
> that we can justify it as a valid user for this proposed RPMB
> subsystem. If you are looking for any further suggestions then please
> let us know.

+1

>
> > > [1] U-Boot has mmc specific implementation
> > >
> > > I think OPTEE-OS has CFG_RPMB_FS_DEV_ID option
> > > CFG_RPMB_FS_DEV_ID=1 for /dev/mmcblk1rpmb,
> >
> > Correct. Note that tee-supplicant will ignore this device ID if --rmb-cid
> > is given and use the specified RPMB instead (the CID is a non-ambiguous way
> > to identify a RPMB device).
> >
> > > but in case if a
> > > system has multiple RPMB devices such as UFS/eMMC/NVMe, one them
> > > should be declared as secure storage and optee should access that one only.
> >
> > Indeed, that would be an equivalent of tee-supplicant's --rpmb-cid.
> >
> > > Sumit, do you have suggestions for this ?
> >
>
> I would suggest having an OP-TEE secure DT property that would provide
> the RPMB CID which is allocated to the secure world.

Another option is for OP-TEE to iterate over all RPMBs with a
programmed key and test if the key OP-TEE would use works. That should
avoid the problem of provisioning a device-unique secure DTB. I'd
expect that the RPMB key is programmed by a trusted provisioning tool
since allowing OP-TEE to program the RPMB key has never been secure,
not unless the OP-TEE binary is rollback protected.

Cheers,
Jens

>
> -Sumit
>
> >
> > --
> > Jerome