Adding EFI runtime support to the Arm's FF-A bus

[Ilias Apalodimas]

Hi Mark, Abdellatif

On Thu, 14 Dec 2023 at 18:47, Mark Kettenis <mark.kettenis at xs4all.nl> wrote:
>
> > Date: Thu, 14 Dec 2023 15:53:46 +0000
> > From: Abdellatif El Khlifi <abdellatif.elkhlifi at arm.com>
>
> Hi Abdellatif,
>
> > Hi guys,
> >
> > I'd like to ask for advice regarding adding EFI RT support to the Arm's FF-A bus
> > in U-Boot.
> >
> > The objective is to enable the FF-A messaging APIs in EFI RT to be
> > used for comms with the secure world. This will help getting/setting
> > EFI variables through FF-A.
> >
> > The existing FF-A APIs in U-Boot call the DM APIs (which are not available at RT).
> >
> > Two possible solutions:
> >
> > 1/ having the entire U-Boot in RT space (as Simon stated in this discussion[1])
>
> I don't think this is a terribly good idea.  With this approach orders
> of magnitude more code will be present in kernel address space one the
> OS kernel is running and calling into the EFI runtime.  Including code
> that may access hardware devices that are now under OS control.  It
> will be nigh impossible to audit all that code and make sure that only
> a safe subset of it gets called.  So...

+100
I think we should draw a line here. I mentioned it on another thread,
but I did a shot BoF in Plumbers discussing issues like this,
problems, and potential solutions [0] [1]. Since that talk patches for
the kernel that 'solve' the problem for RPMBs got pulled into
linux-next [2].
The TL;DR of that talk is that if the kernel ends up being in control
of the hardware that stores the EFI variables, we need to find elegant
ways to teach the kernel how to store those directly. The EFI
requirement of an isolated flash is something that mostly came from
the x86 world and is not a reality on the majority of embedded boards.
I also think we should give up on Authenticated EFI variables in that
case. We get zero guarantees unless the medium has similar properties
to an RPMB.
If a vendor cares about proper UEFI secure boot he can implement
proper hardware.

>
> >
> > 2/ Create an RT variant for the FF-A APIs needed.
> >       These RT variant don't call the DM APIs
> >       (e.g: ffa_mm_communicate_runtime, ffa_sync_send_receive_runtime, ...)
> >
> > What do you recommend please ?
>
> ...this is what I would recommend.  Preferably in a way that refactors
> the code such that the low-level functionality is shared between the
> DM and non-DM APIs.

Yes. The only thing you need to keep alive is the machinery to talk to
the secure world. The bus, flash driver etc should all be running
isolated in there. In that case you can implement SetVariableRT as
described the the EFI spec.

[...]

[0] https://www.youtube.com/watch?v=UdQk0SCUAlA
[1] https://lpc.events/event/17/contributions/1653/attachments/1338/2682/Plumbers%20-%20EFI%20setvariable%20problems%20and%20solutions.pdf
[2] https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=c44b6be62e8dd4ee0a308c36a70620613e6fc55f


/Ilias