Re: [PATCH] arm64/mm: Route faults taken by EFI runtime firmware to no_context

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

On Fri, 9 Oct 2026, at 16:34, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@kernel.org>
>
> When EFI runtime services are invoked from a preemptible kthread, the
> kthread runs with efi_mm installed via kthread_use_mm() and preemption
> enabled, so neither faulthandler_disabled() nor !mm applies in
> do_page_fault(). A permission fault taken by the firmware on one of its
> own (TTBR0) mappings then hits the 'access to user memory outside
> uaccess routines' check and oopses, without efi_runtime_fixup_exception()
> ever being consulted.
>
> efi_mm has no VMAs, so there is nothing for the mm to resolve. Send such
> faults to no_context directly, so that __do_kernel_fault() can recover
> from them as it does when the call is made with preemption disabled.
>
> Fixes: a5baf582f4c0 ("arm64/efi: Call EFI runtime services without 
> disabling preemption")
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  arch/arm64/mm/fault.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
>
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index 0b52557652be..db308fe01c4f 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -40,6 +40,7 @@
>  #include <asm/kprobes.h>
>  #include <asm/mte.h>
>  #include <asm/processor.h>
> +#include <asm/stacktrace.h>
>  #include <asm/sysreg.h>
>  #include <asm/system_misc.h>
>  #include <asm/tlbflush.h>
> @@ -621,6 +622,17 @@ static int __kprobes do_page_fault(unsigned long 
> far, unsigned long esr,
>  	if (faulthandler_disabled() || !mm)
>  		goto no_context;
> 
> +	/*
> +	 * Faults taken by EFI runtime services firmware are never resolved by
> +	 * the mm (efi_mm has no VMAs), and must reach __do_kernel_fault() so
> +	 * that efi_runtime_fixup_exception() can recover from them. When EFI
> +	 * runtime services are invoked from a preemptible kthread, neither of
> +	 * the conditions above applies, so check for this explicitly.
> +	 */
> +	if (IS_ENABLED(CONFIG_EFI) && !user_mode(regs) &&
> +	    is_ttbr0_addr(regs->pc) && current_in_efi())
> +		goto no_context;
> +

Looking at this again, I wonder if we should just add a comparison with
&efi_mm to the preceding if() condition.