[PATCH] arm64/efi: Avoid voluntary preemption with efi_mm installed

[Ard Biesheuvel]

On Tue, 11 Aug 2026, at 16:04, Will Deacon wrote:
> Gus reports a bad kernel memory access when using software PAN
> (CONFIG_ARM64_SW_TTBR0_PAN=y) on a machine with support for EFI runtime
> services:
>
>   Unable to handle kernel access to user memory outside uaccess routines
>     at virtual address 00000000f322ff30
>   Mem abort info:
>     ESR = 0x0000000096000004
>     FSC = 0x04: level 0 translation fault
>   Internal error: Oops: 0000000096000004 [#1]  SMP
>   Workqueue: efi_rts_wq efi_call_rts
>   pstate: 80400005 (Nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>   pc : efi_call_rts+0xd8/0x288
>   Call trace:
>    efi_call_rts+0xd8/0x288 (P)
>    process_one_work+0x178/0x4f8
>    worker_thread+0x194/0x328
>
> This is because the fpsimd context management code called from
> __efi_fpsimd_begin() can preempt voluntarily, returning later to the EFI
> code with an incorrect value for TTBR0_EL1 thanks to the deferred mm
> switching used by the software PAN implementation.
>
> Since EFI runtime services cannot preempt voluntarily and because the
> fpsimd switching code does not rely on the TTBR0_EL1 mappings, simply
> reorder the fpsimd switch so that it occurs before we change the
> page-table.
>
> Cc: Ard Biesheuvel <ardb at kernel.org>
> Reported-by: Gus Bourg <gus at bourg.net>
> Tested-by: Gus Bourg <gus at bourg.net>
> Fixes: a5baf582f4c0 ("arm64/efi: Call EFI runtime services without 
> disabling preemption")
> Link: 
> https://lore.kernel.org/all/20260806000144.3388823-1-gus@bourg.net/
> Signed-off-by: Will Deacon <will at kernel.org>

Reviewed-by: Ard Biesheuvel <ardb at kernel.org>