linux-kernel - Re: [RFC PATCH 1/5] rpmb: add Replay Protected Memory Block (RPMB) subsystem

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Date:   Fri, 5 Mar 2021 09:44:28 +0100
From:   Arnd Bergmann <arnd@...aro.org>
To:     Joakim Bech <joakim.bech@...aro.org>
Cc:     Alex Bennée <alex.bennee@...aro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Maxim Uvarov <maxim.uvarov@...aro.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        ruchika.gupta@...aro.org,
        "Winkler, Tomas" <tomas.winkler@...el.com>, yang.huang@...el.com,
        bing.zhu@...el.com, Matti.Moell@...nsynergy.com,
        hmo@...nsynergy.com, linux-mmc <linux-mmc@...r.kernel.org>,
        linux-scsi <linux-scsi@...r.kernel.org>,
        linux-nvme@...r.kernel.org, Ulf Hansson <ulf.hansson@...aro.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Arnd Bergmann <arnd.bergmann@...aro.org>
Subject: Re: [RFC PATCH 1/5] rpmb: add Replay Protected Memory Block (RPMB) subsystem

On Fri, Mar 5, 2021 at 8:52 AM Joakim Bech <joakim.bech@...aro.org> wrote:
> On Thu, Mar 04, 2021 at 09:56:24PM +0100, Arnd Bergmann wrote:
> > On Wed, Mar 3, 2021 at 2:54 PM Alex Bennée <alex.bennee@...aro.org> wrote:
> > That said, I can also imagine use cases where we do want to
> > store the key in the kernel's keyring, so maybe we end up needing
> > both.
> >
> The concern I have in those cases is that you need to share the RPMB key
> in some way if you need to access the RPMB device from secure side as
> well as from the non-secure side. Technically doable I guess, but in
> practice and in terms of security it doesn't seem like a good approach.
>
> In a shared environment like that you also have the problem that you
> need to agree on how to actually store files on the RPMB device. OP-TEE
> has it's own "FAT-look-a-like" implementation when using RPMB. But if
> you need mutual access, then you need to get into agreement on where to
> actually store the files in the RPMB.
>
> However, if secure side for some reason doesn't use RPMB at all, then
> kernel could of course take control of it and use it.
>
> I would probably not spend too much time on taking that use case into
> account until we actually see a real need for it.

I think the scenario for the 'nvme-rpmb' tool that does the signing in user
space does not involve any TEE at the moment, because PCs usually
don't have one.

I agree that sharing the RPMB is not a great idea, so if you have a TEE
in the system that requires an RPMB for storage, it won't be usable by
anything else. However, you can have multiple RPMB partitions with separate
keys on an NVMe drive, and you can easily have multiple emulated
virtio-rpmb devices  in a guest and use them for purposes other than the
TEE.

      Arnd