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

[Abdellatif El Khlifi]

Hi Ilias

On Thu, Dec 14, 2023 at 09:47:13PM +0200, Ilias Apalodimas 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].

I watched your talk. Great work, thanks :)

> 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.

Cool, thanks. That's my preferred solution too.

mm_communicate() should be able to detect runtime mode so it calls ffa_mm_communicate_runtime().

Is there a way to check whether we are in EFI runtime or not ?

Suggested changes (pseudo-code):

__efi_runtime mm_communicate () {
#if CONFIG_IS_ENABLED(ARM_FFA_TRANSPORT)
if (RT) { /* NEW */
        ret = ffa_mm_communicate_runtime(comm_buf, dsize); /* NEW */
} else {
    mm_comms = get_mm_comms();
    if (mm_comms == MM_COMMS_FFA)
        ret = ffa_mm_communicate(comm_buf, dsize);
    else
        ret = optee_mm_communicate(comm_buf, dsize);
}
#else
...
#endif

Existing code:  https://github.com/u-boot/u-boot/blob/master/lib/efi_loader/efi_variable_tee.c#L417

Cheers,
Abdellatif