Hi,
On Thu, 14 Dec 2023 at 12:47, Ilias Apalodimas ilias.apalodimas@linaro.org wrote:
Hi Mark, Abdellatif
On Thu, 14 Dec 2023 at 18:47, Mark Kettenis mark.kettenis@xs4all.nl wrote:
Date: Thu, 14 Dec 2023 15:53:46 +0000 From: Abdellatif El Khlifi abdellatif.elkhlifi@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/Plumber... [2] https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?...
Regards, Simon