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

[Ilias Apalodimas]

On Mon, 21 Aug 2023 at 14:19, Jens Wiklander <jens.wiklander@xxxxxxxxxx> wrote:
>
> 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.

+1 to that.  Overall we shound't 'trust' to do the programming. For
example, in OP-TEE if you compile it with device programming
capabilities, you can easily convince OP-TEE to send you the symmetric
key by swapping the supplicant with a malicious application.

Thanks
/Ilias
>
> Cheers,
> Jens
>
> >
> > -Sumit
> >
> > >
> > > --
> > > Jerome