Re: [PATCH] x86: efi: avoid BUILD_BUG_ON() for non-constant p4d_index

[Ard Biesheuvel]

On Thu, 4 Feb 2021 at 11:52, Borislav Petkov <bp@xxxxxxxxx> wrote:
>
> On Wed, Feb 03, 2021 at 09:29:18PM +0100, Ard Biesheuvel wrote:
> > I think we have agreement on the approach but it is unclear who is
> > going to write the patch.
>
> How's that below?
>
> And frankly, I'd even vote for removing those assertions altogether. If
> somehow the EFI pgd lands somewhere else, the kernel will crash'n'burn
> spectacularly and quickly so it's not like we won't catch it...
>

+1 for just removing them

> ---
> diff --git a/arch/x86/include/asm/pgtable_64_types.h b/arch/x86/include/asm/pgtable_64_types.h
> index 91ac10654570..b6be19c09841 100644
> --- a/arch/x86/include/asm/pgtable_64_types.h
> +++ b/arch/x86/include/asm/pgtable_64_types.h
> @@ -156,8 +156,8 @@ extern unsigned int ptrs_per_p4d;
>  #define CPU_ENTRY_AREA_PGD     _AC(-4, UL)
>  #define CPU_ENTRY_AREA_BASE    (CPU_ENTRY_AREA_PGD << P4D_SHIFT)
>
> -#define EFI_VA_START           ( -4 * (_AC(1, UL) << 30))
> -#define EFI_VA_END             (-68 * (_AC(1, UL) << 30))
> +#define EFI_VA_START           ( -4UL * (_AC(1, UL) << 30))
> +#define EFI_VA_END             (-68UL * (_AC(1, UL) << 30))
>
>  #define EARLY_DYNAMIC_PAGE_TABLES      64
>
> diff --git a/arch/x86/platform/efi/efi_64.c b/arch/x86/platform/efi/efi_64.c
> index e1e8d4e3a213..56fdc0bbb554 100644
> --- a/arch/x86/platform/efi/efi_64.c
> +++ b/arch/x86/platform/efi/efi_64.c
> @@ -123,9 +123,7 @@ void efi_sync_low_kernel_mappings(void)
>          * only span a single PGD entry and that the entry also maps
>          * other important kernel regions.
>          */
> -       MAYBE_BUILD_BUG_ON(pgd_index(EFI_VA_END) != pgd_index(MODULES_END));
> -       MAYBE_BUILD_BUG_ON((EFI_VA_START & PGDIR_MASK) !=
> -                       (EFI_VA_END & PGDIR_MASK));
> +       MAYBE_BUILD_BUG_ON((EFI_VA_START & PGDIR_MASK) != PGDIR_MASK);
>
>         pgd_efi = efi_pgd + pgd_index(PAGE_OFFSET);
>         pgd_k = pgd_offset_k(PAGE_OFFSET);
> @@ -137,8 +135,7 @@ void efi_sync_low_kernel_mappings(void)
>          * As with PGDs, we share all P4D entries apart from the one entry
>          * that covers the EFI runtime mapping space.
>          */
> -       BUILD_BUG_ON(p4d_index(EFI_VA_END) != p4d_index(MODULES_END));
> -       BUILD_BUG_ON((EFI_VA_START & P4D_MASK) != (EFI_VA_END & P4D_MASK));
> +       BUILD_BUG_ON((EFI_VA_START & P4D_MASK) != P4D_MASK);
>
>         pgd_efi = efi_pgd + pgd_index(EFI_VA_END);
>         pgd_k = pgd_offset_k(EFI_VA_END);
>
>
> --
> Regards/Gruss,
>     Boris.
>
> https://people.kernel.org/tglx/notes-about-netiquette