Re: [PATCH 1/2] arm64: efi: Execute runtime services from a dedicated stack

Ard Biesheuvel
2022-12-09T10:53:23Z

On Fri, 9 Dec 2022 at 11:51, Mark Rutland <mark.rutland@arm.com> wrote:
>
> Hi Ard,
>
> One drive-by comment below...
>
> On Mon, Dec 05, 2022 at 09:12:09PM +0100, Ard Biesheuvel wrote:
> > With the introduction of PRMT in the ACPI subsystem, the EFI rts
> > workqueue is no longer the only caller of efi_call_virt_pointer() in the
> > kernel. This means the EFI runtime services lock is no longer sufficient
> > to manage concurrent calls into firmware, but also that firmware calls
> > may occur that are not marshalled via the workqueue mechanism, but
> > originate directly from the caller context.
> >
> > For added robustness, and to ensure that the runtime services have 8 KiB
> > of stack space available as per the EFI spec, introduce a spinlock
> > protected EFI runtime stack of 8 KiB, where the spinlock also ensures
> > serialization between the EFI rts workqueue (which itself serializes EFI
> > runtime calls) and other callers of efi_call_virt_pointer().
> >
> > While at it, use the stack pivot to avoid reloading the shadow call
> > stack pointer from the ordinary stack, as doing so could produce a
> > gadget to defeat it.
> >
> > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> > ---
> > arch/arm64/include/asm/efi.h | 3 +++
> > arch/arm64/kernel/efi-rt-wrapper.S | 13 +++++++++-
> > arch/arm64/kernel/efi.c | 25 ++++++++++++++++++++
>
> We'll need to teach the stack unwinder about this, or if we take an exception
> from the EFI stack, the backtrace will terminate as soon as it hits a frame
> record on the EFI stack.
>
> In arch/arm64/kernel/stacktrace.c's arch_stack_walk(), that'll need to be added
> to the array of stack bounds. Ideally we'd only add that when a thread is
> making an EFI call.
>

Thanks, I'll look into that.