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

["Ard Biesheuvel" <ardb@kernel.org>]

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 <gus@bourg.net>
>> 
>> 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);
 }