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

[Simon Glass]

Hi,

On Thu, 14 Dec 2023 at 12:47, Ilias Apalodimas
<ilias.apalodimas at linaro.org> wrote:
>
> 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.

Just to copy in my thoughts as they are lost at this point:

> We would need to publish a runtime interface with access to the driver
> API. I did ask for this when the EFI runtime support was added, but it
> wasn't done.

> It would be possible to create a new 'runtime' phase of U-Boot (RPL?),
> separate from the others. That will be much easier once we get the XPL
> stuff sorted out., since adding new [hase would be fairly trivial  CPL
> died as another contributor had a series which went in first...then I
> never got back to it.

> So for now having the entire U-Boot in runtime space seems reasonable to me.

> I'll also mention that it would be nice to have s new-style API
> (replacing the old API U-Boot currently has) which uses more of a
> module approach. E.g. we could declare that uclass_first_device() is
> exported and can be called from outside U-Boot.

>
> >
> > >
> > > 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.

The current approach is pretty brittle, since it relies on putting
some of the U-Boot code into a separate area. There is no good way to
know which U-Boot code should be in that area, since we don't create a
separate build. If a function calls one that has not been specially
marked, or accesses data that is not in the area, then it will crash
or hang.

So, as I said, I think we need a new build, if we want to avoid all of
U-Boot in there. Anything else is hard to maintain.

>
> [...]
>
> [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
>

Regards,
Simon