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

[Arvind Sankar]

On Thu, Feb 04, 2021 at 11:51:55AM +0100, Borislav Petkov 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...

Removing altogether should be fine, but see below if we don't.

> 
> ---
> 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))

This doesn't have any effect right? And the reason for the _AC() stuff
in there is to allow the #define to be used in assembler -- this
particular one isn't, but it makes no sense to use the UL suffix as well
as _AC() in the same macro.

>  
>  #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);

This check is superfluous. Just do the P4D one.

>  
>  	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);

This should check EFI_VA_END instead of EFI_VA_START, and maybe throw in
a BUG_ON if EFI_VA_END >= EFI_VA_START.

>  
>  	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