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

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Message-ID: <76da826f-b608-6add-5401-6de818b180e3@siemens.com>
Date:   Wed, 7 Jun 2023 14:40:30 +0200
From:   Jan Kiszka <jan.kiszka@...mens.com>
To:     Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Sumit Garg <sumit.garg@...aro.org>
Cc:     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

On 07.06.23 10:25, Ilias Apalodimas wrote:
> Hi Sumit,
> 
> On Wed, 7 Jun 2023 at 10:25, Sumit Garg <sumit.garg@...aro.org> wrote:
>>
>> 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?
> 
> No I don't, this will work reliably without the need to remount the efivarfs.
> As you point out you will still have this dependency if you end up
> building them as modules and you manage to mount the efivarfs before
> those get inserted.  Does anyone see a reasonable workaround?
> Deceiving the kernel and making the bootloader set the RT property bit
> to force the filesystem being mounted as rw is a nasty hack that we
> should avoid.  Maybe adding a kernel command line parameter that says
> "Ignore the RTPROP I know what I am doing"?  I don't particularly love
> this either, but it's not unreasonable.

In the context of https://github.com/OP-TEE/optee_os/issues/6094,
basically this issue mapped on reboot/shutdown, I would really love to
see the unhandy tee-supplicant daemon to be overcome.

Jan

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