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

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

--->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)