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

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Message-ID: <20210205185622.GA461042@localhost>
Date:   Fri, 5 Feb 2021 11:56:22 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        Ard Biesheuvel <ardb@...nel.org>,
        Arnd Bergmann <arnd@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, X86 ML <x86@...nel.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Darren Hart <dvhart@...radead.org>,
        Andy Shevchenko <andy@...radead.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        linux-efi <linux-efi@...r.kernel.org>,
        platform-driver-x86@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH] x86/efi: Remove EFI PGD build time checks

On Fri, Feb 05, 2021 at 12:39:30PM +0100, Borislav Petkov wrote:
> From: Borislav Petkov <bp@...e.de>
> 
> 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@...db.de>
> Link: http://lkml.kernel.org/r/20210107223424.4135538-1-arnd@kernel.org
> Signed-off-by: Borislav Petkov <bp@...e.de>

Reviewed-by: Nathan Chancellor <nathan@...nel.org>
Tested-by: Nathan Chancellor <nathan@...nel.org>

> ---
>  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