Re: [PATCH] arm64/efi: Do not call EFI runtime services preemptibly under SW PAN

[Will Deacon <will@kernel.org>]

On Mon, Aug 10, 2026 at 05:51:16PM -0700, Gus Bourg wrote:
> On Mon, Aug 10, 2026 at 4:04 AM Will Deacon <will@kernel.org> wrote:
> > On Fri, Aug 07, 2026 at 04:56:00PM +0300, Ard Biesheuvel wrote:
> > > On Fri, 7 Aug 2026, at 15:16, Will Deacon wrote:
> > > > On Wed, Aug 05, 2026 at 05:01:44PM -0700, gus bourg wrote:
> > > >> There is such a preemption point in arch_efi_call_virt_setup() itself,
> > > >> immediately after the TTBR0 install: __efi_fpsimd_begin() ->
> > > >> kernel_neon_begin() -> put_cpu_fpsimd_context() -> local_bh_enable() ->
> > > >> preempt_check_resched().
> > > >
> > > > Hmm, can we move the ttbr0 toggling until after __efi_fpsimd_begin() so
> > > > that this preemption point doesn't interfere?
> > > >
> > >
> > > Either that, or tweak the logic so that the switch is done eagerly in
> > > this particular case (untested):
> > >
> > > --- a/arch/arm64/mm/context.c
> > > +++ b/arch/arm64/mm/context.c
> > > @@ -266,7 +266,7 @@
> > >          * Defer TTBR0_EL1 setting for user threads to uaccess_enable() when
> > >          * emulating PAN.
> > >          */
> > > -       if (!system_uses_ttbr0_pan())
> > > +       if (!system_uses_ttbr0_pan() || mm_is_efi(mm))
> > >                 cpu_switch_mm(mm->pgd, mm);
> > >  }
> >
> > I think I'd prefer to avoid special-casing this, if we can. I was just
> > thinking of the diff below.
> 
> Tested the v2 approach (__efi_fpsimd_begin() before
> uaccess_ttbr0_enable()) on the board that originally reproduced the
> oops: Khadas VIM3, Amlogic A311D (4x Cortex-A73 + 2x Cortex-A53, no
> FEAT_PAN, so CONFIG_ARM64_SW_TTBR0_PAN is in use), EDK2-based UEFI
> firmware, v7.0.12 plus this change alone (my prior workaround removed).
> 
> Both firmware description modes were exercised (DeviceTree and ACPI
> boots of the same system):
> 
>   - 3,000,000 GetVariable calls (three concurrent readers) plus
>     200,000 GetTime calls per mode, under sustained CPU contention
>     (busy-loop load on half the cores to force scheduling in the call
>     window) - i.e. 6.4M calls total across the ACPI and DeviceTree
>     boots of the same system
>   - SetVariable create/write/delete round-trips in both modes
>   - ~15 reboots/power cycles (ResetSystem) and clean shutdowns across
>     the session
> 
> Zero efi_call_rts faults and zero "Unable to handle kernel access to
> user memory" events; efivars and the EFI RTC remained functional
> throughout, and poweroff/reboot stayed clean. Under the unpatched
> preemptible path the same board faulted within thousands of calls with
> comparable contention, so this comfortably clears the reproduction
> threshold.
> 
> Tested-by: gus bourg <gus@bourg.net>

Thanks for giving it a spin. I'll send it out as a proper patch.

Will