Re: [PATCH] x86/efi: Remove EFI PGD build time checks

[Ard Biesheuvel]

On Fri, 5 Feb 2021 at 12:39, Borislav Petkov <bp@xxxxxxxxx> wrote:
>
> From: Borislav Petkov <bp@xxxxxxx>
>
> With CONFIG_X86_5LEVEL, CONFIG_UBSAN and CONFIG_UBSAN_UNSIGNED_OVERFLOW
> enabled, clang fails the build with
>
>   x86_64-linux-ld: arch/x86/platform/efi/efi_64.o: in function `efi_sync_low_kernel_mappings':
>   efi_64.c:(.text+0x22c): undefined reference to `__compiletime_assert_354'
>
> which happens due to -fsanitize=unsigned-integer-overflow being enabled:
>
>   -fsanitize=unsigned-integer-overflow: Unsigned integer overflow, where
>   the result of an unsigned integer computation cannot be represented
>   in its type. Unlike signed integer overflow, this is not undefined
>   behavior, but it is often unintentional. This sanitizer does not check
>   for lossy implicit conversions performed before such a computation
>   (see -fsanitize=implicit-conversion).
>
> and that fires when the (intentional) EFI_VA_START/END defines overflow
> an unsigned long, leading to the assertion expressions not getting
> optimized away (on GCC they do)...
>
> However, those checks are superfluous: the runtime services mapping
> code already makes sure the ranges don't overshoot EFI_VA_END as the
> EFI mapping range is hardcoded. On each runtime services call, it is
> switched to the EFI-specific PGD and even if mappings manage to escape
> that last PGD, this won't remain unnoticed for long.
>
> So rip them out.
>
> See https://github.com/ClangBuiltLinux/linux/issues/256 for more info.
>
> Reported-by: Arnd Bergmann <arnd@xxxxxxxx>
> Link: http://lkml.kernel.org/r/20210107223424.4135538-1-arnd@xxxxxxxxxx
> Signed-off-by: Borislav Petkov <bp@xxxxxxx>

Acked-by: Ard Biesheuvel <ardb@xxxxxxxxxx>

> ---
>  arch/x86/platform/efi/efi_64.c | 19 -------------------
>  1 file changed, 19 deletions(-)
>
> diff --git a/arch/x86/platform/efi/efi_64.c b/arch/x86/platform/efi/efi_64.c
> index e1e8d4e3a213..8efd003540ca 100644
> --- a/arch/x86/platform/efi/efi_64.c
> +++ b/arch/x86/platform/efi/efi_64.c
> @@ -115,31 +115,12 @@ void efi_sync_low_kernel_mappings(void)
>         pud_t *pud_k, *pud_efi;
>         pgd_t *efi_pgd = efi_mm.pgd;
>
> -       /*
> -        * We can share all PGD entries apart from the one entry that
> -        * covers the EFI runtime mapping space.
> -        *
> -        * Make sure the EFI runtime region mappings are guaranteed to
> -        * 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));
> -
>         pgd_efi = efi_pgd + pgd_index(PAGE_OFFSET);
>         pgd_k = pgd_offset_k(PAGE_OFFSET);
>
>         num_entries = pgd_index(EFI_VA_END) - pgd_index(PAGE_OFFSET);
>         memcpy(pgd_efi, pgd_k, sizeof(pgd_t) * num_entries);
>
> -       /*
> -        * 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));
> -
>         pgd_efi = efi_pgd + pgd_index(EFI_VA_END);
>         pgd_k = pgd_offset_k(EFI_VA_END);
>         p4d_efi = p4d_offset(pgd_efi, 0);
> --
> 2.29.2
>
> --
> Regards/Gruss,
>     Boris.
>
> https://people.kernel.org/tglx/notes-about-netiquette