Re: [PATCH 4/4] dt-bindings: firmware: Add Qualcomm UEFI Secure Application client

Ilias Apalodimas
2022-07-28T06:04:23Z

Hi all,

On Wed, 27 Jul 2022 at 16:24, Sudeep Holla <sudeep.holla@arm.com> wrote:
>
> On Wed, Jul 27, 2022 at 03:03:49PM +0200, Maximilian Luz wrote:
> >
> > Is there really a good way around it?
>
> Yes rely on the firmware preferably auto discover, if that is not an option,
> how about query. It seem to be working in your case.

That's a good point. We have a similar situation with some Arm
devices and U-Boot. Let me try to explain a bit.

There's code plugged in in OP-TEE and U-Boot atm which allows you to
store EFI variables on an RPMB. This is a nice alternative if your
device doesn't have any other secure storage, however it presents
some challenges after ExitBootServices, similar to the ones you have
here.

The eMMC controller usually lives in the non-secure world. OP-TEE
can't access that, so it relies on a userspace supplicant to perform
the RPMB accesses. That supplicant is present in U-Boot and
Get/SetVariable works fine before ExitBootServices. Once Linux boots,
the 'U-Boot supplicant' goes away and we launch the linux equivalent
one from userspace. Since variable accessing is a runtime service and
it still has to go through the firmware we can't use those anymore
since U-Boot doesn't preserve the supplicant, the eMMC driver and the
OP-TEE portions needed in the runtime section(and even if it did we
would now have 2 drivers racing to access the same hardware). Instead
U-Boot copies the variables in runtime memory and
GetVariable/GetNextVariable still works, but SetVariable returns
EFI_UNSUPPORTED.

I've spent enough time looking at available solutions and although
this indeed breaks the EFI spec, something along the lines of
replacing the runtime services with ones that give you direct access
to the secure world, completely bypassing the firmware is imho our
least bad option.

I have an ancient branch somewhere that I can polish up and send an
RFC [1], but the way I enabled that was to install an empty config
table from the firmware. That empty table is basically an indication
to the kernel saying "Hey I can't store variables, can you do that for
me".

Is there any chance we can do something similar on that device (or
find a reasonable way of inferring that we need to replace some
services). That way we could at least have a common entry point to
the kernel and leave out the DT changes.

[1] https://git.linaro.org/people/ilias.apalodimas/net-next.git/log/?h=setvar_rt_optee_3

Thanks
/Ilias

>
> > As far as I can see the alternative (especially for the apps that
> > need to be loaded manually) is hard-coding everything in the driver.
> > Which IMHO just spreads device specific information everywhere.
> >
>
> It may not be too bad compared to putting loads of firmware details
> in the DT. What happens if you get a firmware upgrade with changed
> number of firmware entities or even if the names are changed.
>
> Are these name user ABI in a way that they won't be changed ? Generally
> these entities tend to use UUID and the name you have might get changed.
>
> I would ideally prefer even the name to be supplied from the userspace.
> In this particular case, make this a driver and have the name as the
> parameter. If the secure side services are used by some non-secure
> applications, then you will need to have a user-interface which means
> you can get the named from the userspace. No need to change the driver
> in either case. Please let me know if I am missing anything to consider
> here.
>
> > Also: Let's use the TPM app as example. If that would be a SPI or I2C
> > device, you'd model it in the DT. Just because it's a hardware device
> > that's accessible via SCM/firmware you now don't?
> >
>
> Not sure if I understand the comparison here. But if there is some device
> that is access restricted but needs to be accessed and has mechanism to
> access, then you would model it as device in DT.
>
> But the one $subject is addressing looks pure software and doesn't make
> sense to model in DT IMO.
>
> > If I were absolutely certain that there is a reliable mechanism to
> > detect these apps, I'd agree with having a driver to instantiate those
> > devices. But I am not.
> >
>
> You did say you use some query API to check this. I haven't seen the driver,
> so relying on what you said earlier.
>
> --
> Regards,
> Sudeep