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

On Mon, Aug 10, 2026 at 4:04 AM Will Deacon <[email protected]> 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:
> > >> From: Gus Bourg <[email protected]>
> > >>
> > >> When PAN is emulated by switching TTBR0_EL1, arch_efi_call_virt_setup()
> > >> installs the EFI mm into TTBR0_EL1 via uaccess_ttbr0_enable(), and only a
> > >> return from exception ever puts it back: check_and_switch_context() skips
> > >> the register write by design ("Defer TTBR0_EL1 setting for user threads to
> > >> uaccess_enable() when emulating PAN"), and __switch_to() does not touch it
> > >> either. switch_mm() updates only thread_info->ttbr0.
> > >>
> > >> Since commit a5baf582f4c0 ("arm64/efi: Call EFI runtime services without
> > >> disabling preemption") the runtime call is preemptible, so the EFI worker
> > >> can now be scheduled out inside that window. An involuntary preemption is
> > >> harmless, because __swpan_entry_el1()/__swpan_exit_el1() save and restore
> > >> the state around the exception. A voluntary reschedule is not: it performs
> > >> no return from exception, so the worker resumes with another task's
> > >> TTBR0_EL1 - or reserved_pg_dir - and the next efi_mm access takes a level 0
> > >> translation fault.
> > >>
> > >> 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.
>
> Will

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 <[email protected]>


>
> --->8
>
> diff --git a/arch/arm64/kernel/efi.c b/arch/arm64/kernel/efi.c
> index 30cd7f804398..0ec90fd1754e 100644
> --- a/arch/arm64/kernel/efi.c
> +++ b/arch/arm64/kernel/efi.c
> @@ -184,6 +184,8 @@ void arch_efi_call_virt_setup(void)
>                 efi_virtmap_load();
>         }
>
> +       __efi_fpsimd_begin();
> +
>         /*
>          * Enable access to the valid TTBR0_EL1 and invoke the errata
>          * workaround directly since there is no return from exception when
> @@ -191,8 +193,6 @@ void arch_efi_call_virt_setup(void)
>          */
>         uaccess_ttbr0_enable();
>         post_ttbr_update_workaround();
> -
> -       __efi_fpsimd_begin();
>  }
>
>  void arch_efi_call_virt_teardown(void)
>