linux-kernel - Re: [PATCH v5 3/3] efi: Add tee-based EFI variable driver

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Message-ID: <CAFA6WYNFYB1LiOFB_iwTsdD5PmnDdSbtDSH2J4FVFPx3uik8rQ@mail.gmail.com>
Date:   Wed, 7 Jun 2023 12:55:19 +0530
From:   Sumit Garg <sumit.garg@...aro.org>
To:     Ilias Apalodimas <ilias.apalodimas@...aro.org>
Cc:     Jan Kiszka <jan.kiszka@...mens.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Masahisa Kojima <masahisa.kojima@...aro.org>,
        Jens Wiklander <jens.wiklander@...aro.org>,
        linux-kernel@...r.kernel.org, op-tee@...ts.trustedfirmware.org,
        Johan Hovold <johan+linaro@...nel.org>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        linux-efi@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org,
        "Su, Bao Cheng (RC-CN DF FA R&D)" <baocheng.su@...mens.com>
Subject: Re: [PATCH v5 3/3] efi: Add tee-based EFI variable driver

Hi Ilias,

On Wed, 7 Jun 2023 at 12:05, Ilias Apalodimas
<ilias.apalodimas@...aro.org> wrote:
>
> Hi Jan,
>
> [...]
>
> > >>>>
> > > ...
> > >>>
> > >>> I think we have a probe ordering issue with this driver:
> > >>> efivarfs_fill_super() may be called before the TEE bus was probed, thus
> > >>> with the default efivar ops still registered. And that means
> > >>> efivar_supports_writes() will return false, and the fs declares itself
> > >>> as readonly. I've seen systemd mounting it r/o initialling, and you need
> > >>> to remount the fs to enable writability.
> > >>>
> > >>> Is there anything that could be done to re-order things reliably, probe
> > >>> the tee bus earlier etc.?
> > >>
> > >> This driver has a dependency on user-space daemon: tee-supplicant to
> > >> be running for RPMB access. So once you start that daemon the
> > >> corresponding device will be enumerated on the TEE bus and this driver
> > >> probe will be invoked. So I would suggest you to load this daemon very
> > >> early in the boot process or better to make it a part of initramfs.
> > >>
> > >
> > > That is not the point, really.
> > >
> > > If this dependency exists, the code should be aware of that, and made
> > > to work correctly in spite of it. Requiring a module to be part of
> > > initramfs is not a reasonable fix.
> >
> > In fact, I've tested a non-modularized build as well, just to exclude
> > that issue. The daemon dependency is more likely the problem here.
> >
> > >
> > > IIUC, this also means that the efivar ops are updated while there is
> > > already a client. This seems less than ideal as well
>
> As Sumit pointed out, the 'device' won't be available from OP-TEE
> until the supplicant is up and running and as a result, the module
> _probe() function won't run.  Unfortunately, this isn't something we
> can avoid since the supplicant is responsible for the RPMB writes.
> The only thing I can think of is moving parts of the supplicant to the
> kernel and wiring up the RPC calls for reading/writing data to the
> eMMC subsystem.  There was another discussion here [0] requesting the
> same thing for different reasons. But unless I am missing something
> this won't solve the problem completely either.  You still have a
> timing dependency of "when did the RT callbacks change" -- "when was
> my efivarfs mounted".

With the RPMB writes wired through the kernel [1], the only dependency
left is when do you load the tee-stmm-efi driver to have real EFI
runtime variables support. IMO, tee-stmm-efi driver should be built-in
to support systems without initramfs. The distro installers may choose
to bundle it in initramfs. Do you still see a timing dependency with
this approach?

[1] Of Course here we need the eMMC and TEE/OPTEE drivers to be built-in too.

-Sumit

>
> Thanks
> /Ilias
> >
> > Jan
> >
> > --
> > Siemens AG, Technology
> > Competence Center Embedded Linux
> >