Re: [PATCH] x86/mm+efi: Avoid creating W+X mappings

[Ard Biesheuvel]

On Thu, 22 Sept 2022 at 21:32, Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> wrote:
>
> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>
> I'm planning on sticking this in x86/mm so that it goes upstream
> along with the W+X detection code.
>
> --
>
> A recent x86/mm change warns and refuses to create W+X mappings.
>
> The 32-bit EFI code tries to create such a mapping and trips over
> the new W+X refusal.
>
> Make the EFI_RUNTIME_SERVICES_CODE mapping read-only to fix it.
>

This is not safe. EFI_RUNTIME_SERVICES_CODE covers both .text and
.data sections of the EFI runtime PE/COFF executables in memory, so
you are essentially making .data and .bss read-only. (Whether those
executables actually modify their .data and .bss at runtime is a
different matter, but the point is that it used to be possible)

More recent firmwares may provide a 'memory attributes table'
separately which describes the individual sections, but older 32-bit
firmwares are not even built with 4k section alignment, so code and
data may share a single page. Note that we haven't wired up this
memory attributes table on i386 at the moment, and I seriously doubt
that 32-bit firmware in the field exposes it.

Can we just turn off this feature for 32-bit?

> Reported-by: Guenter Roeck <linux@xxxxxxxxxxxx>
> Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Tested-by: Guenter Roeck <linux@xxxxxxxxxxxx>
> Cc: Ard Biesheuvel <ardb@xxxxxxxxxx>
> Cc: Darren Hart <dvhart@xxxxxxxxxxxxx>
> Cc: Andy Shevchenko <andy@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: linux-efi@xxxxxxxxxxxxxxx
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Link: https://lore.kernel.org/all/d8cd7c7e-24c1-7f70-24a9-91c77aa634af@xxxxxxxxxxxx/
> ---
>  arch/x86/platform/efi/efi_32.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/arch/x86/platform/efi/efi_32.c b/arch/x86/platform/efi/efi_32.c
> index e06a199423c0..d81e379fcd43 100644
> --- a/arch/x86/platform/efi/efi_32.c
> +++ b/arch/x86/platform/efi/efi_32.c
> @@ -136,6 +136,7 @@ void __init efi_runtime_update_mappings(void)
>                         if (md->type != EFI_RUNTIME_SERVICES_CODE)
>                                 continue;
>
> +                       set_memory_ro(md->virt_addr, md->num_pages);
>                         set_memory_x(md->virt_addr, md->num_pages);
>                 }
>         }