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

Ilias Apalodimas
2022-07-28T12:36:00Z

Hi Maximilian

On Thu, 28 Jul 2022 at 13:48, Maximilian Luz <luzmaximilian@gmail.com> wrote:
>
> On 7/28/22 08:03, Ilias Apalodimas wrote:
> > 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.
>
> This sounds very similar to what Qualcomm may be doing on some devices.
> The TrEE interface allows for callbacks and there are indications that
> one such callback-service is for RPMB. I believe that at least on some
> platforms, Qualcomm also stores UEFI variables in RPMB and uses the same
> uefisecapp interface in combination with RPMB listeners installed by the
> kernel to access them.
>
> > 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
>
> I would very much like to avoid the need for special bootloaders. The
> devices we're talking about are WoA devices, meaning they _should_
> ideally boot just fine with EFI and ACPI.

I've already responded to following email, but I'll repeat it here for
completeness. It's not a special bootloader. It's the opposite, it's
a generic UEFI compliant bootloader which takes advantage of the fact
EFI is extensible. We are doing something very similar in how we load
our initrd via the EFI_LOAD_FILE2 protocol. Whether Qualcomm can add
that to their bootloaders is a different topic though. But at some
point we need to draw a line than keep overloading the DT because a
vendor decided to go down it's own path.

>
> From an end-user perspective, it's annoying enough that we'll have to
> stick with DTs for the time being due to the use of PEPs in ACPI. I
> really don't want to add some special bootloader for fixups to that.
> Also, this would just move the problem from kernel to bootloader.

But it *is* a bootloader problem. The bootloader is aware of the fact
that it can't provide runtime services for X reasons and that's
exactly why we are trying to set EFI_RT_PROPERTIES_TABLE correctly
from the firmware. All we are doing is install a config table to tell
the OS "I can't do that, can you find a way around it?".

Regards
/Ilias

>
> If you have any suggestions for another way of detecting this, please
> feel free to share. I, unfortunately, don't.
>
> Regards,
> Max