[PATCH v3 2/2] arm64/acpi: disallow writeable AML opregion mapping for EFI code regions

[Shawn Guo]

Hi Ard,

On Fri, Jun 26, 2020 at 05:58:32PM +0200, Ard Biesheuvel wrote:
> Given that the contents of EFI runtime code and data regions are
> provided by the firmware, as well as the DSDT, it is not unimaginable
> that AML code exists today that accesses EFI runtime code regions using
> a SystemMemory OpRegion. There is nothing fundamentally wrong with that,
> but since we take great care to ensure that executable code is never
> mapped writeable and executable at the same time, we should not permit
> AML to create writable mapping.
> 
> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>

I'm booting Lenovo Flex 5G laptop with ACPI, and seeing this change
causes a memory abort[1] when upgrading ACPI tables via initrd[2].
Dropping this change seems to fix the issue for me.  But does that
looks like a correct fix to you?

Shawn

[1] https://fileserver.linaro.org/s/iDe9SaZeNNkyNxG
[2] Documentation/admin-guide/acpi/initrd_table_override.rst

> ---
>  arch/arm64/kernel/acpi.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/arch/arm64/kernel/acpi.c b/arch/arm64/kernel/acpi.c
> index 01b861e225b0..455966401102 100644
> --- a/arch/arm64/kernel/acpi.c
> +++ b/arch/arm64/kernel/acpi.c
> @@ -301,6 +301,15 @@ void __iomem *acpi_os_ioremap(acpi_physical_address phys, acpi_size size)
>  			pr_warn(FW_BUG "requested region covers kernel memory @ %pa\n", &phys);
>  			return NULL;
>  
> +		case EFI_RUNTIME_SERVICES_CODE:
> +			/*
> +			 * This would be unusual, but not problematic per se,
> +			 * as long as we take care not to create a writable
> +			 * mapping for executable code.
> +			 */
> +			prot = PAGE_KERNEL_RO;
> +			break;
> +
>  		case EFI_ACPI_RECLAIM_MEMORY:
>  			/*
>  			 * ACPI reclaim memory is used to pass firmware tables
> -- 
> 2.27.0