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From:   Linus Walleij <linus.walleij@linaro.org>
Date:   Mon, 8 Mar 2021 17:20:08 +0100
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Subject: Re: [RFC PATCH 1/5] rpmb: add Replay Protected Memory Block (RPMB) subsystem
To:     Arnd Bergmann <arnd@linaro.org>, keyrings@vger.kernel.org,
        David Howells <dhowells@redhat.com>,
        Jarkko Sakkinen <jarkko@kernel.org>
Cc:     Joakim Bech <joakim.bech@linaro.org>,
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        Maxim Uvarov <maxim.uvarov@linaro.org>,
        Ilias Apalodimas <ilias.apalodimas@linaro.org>,
        ruchika.gupta@linaro.org,
        "Winkler, Tomas" <tomas.winkler@intel.com>, yang.huang@intel.com,
        bing.zhu@intel.com, Matti.Moell@opensynergy.com,
        hmo@opensynergy.com, linux-mmc <linux-mmc@vger.kernel.org>,
        linux-scsi <linux-scsi@vger.kernel.org>,
        linux-nvme@vger.kernel.org, Ulf Hansson <ulf.hansson@linaro.org>,
        Arnd Bergmann <arnd.bergmann@linaro.org>,
        Hector Martin <marcan@marcan.st>
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On Fri, Mar 5, 2021 at 9:44 AM Arnd Bergmann <arnd@linaro.org> wrote:

> 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.

Isn't that because (Windows-)PC:s prefer to use TPMs which
include their own key storage?

Apple has their "secure enclave" (separate security chip) and as illustrated
by Marcan it did not make use of RPMB as of 2016:
https://marcan.st/2016/03/untangling-ios-pin-code-security/
Maybe they have since started to use it? (They should.)

AFAICT the use case for RPMB is:
1. Used by Android with some TEE, and if you're not running
    Android and some TEE then
2. Use it for whatever you like

As it seems neither Microsoft nor Apple is paying it much attention
(+/- new facts) it will be up to the community to define use cases
for RPMB. I don't know what would make most sense, but the
kernel keyring seems to make a bit of sense as it is a well maintained
keyring project.

So the proposal is to (as some goal) bridge the keyring subsystem
to the proposed RPMB subsystem with an kernel-internal API.

What do the keyring people think of this? Added David & Jarkko to the
thread to get some input.

I suppose it would be a bit brutal if the kernel would just go in and
appropriate any empty RPMB it finds, but I suspect it is the right way
to make use of this facility given that so many of them are just sitting
there unused. Noone will run $CUSTOM_UTILITY any more than they
run the current RPMB tools in mmc-tools.

Agreeing with OP-TEE on the format and management of
any one RPMB partition seems like a good idea, if for nothing else
then for sharing documentation.

> 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.

The eMMC RPMB code even handles multiple RPMB partitions on an
eMMC. But I don't think I have ever seen a device with more than
one RPMB.

Yours,
Linus Walleij