linux-hardening - Re: [PATCH 04/16] x86/boot: Increase boot page table size

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Date:   Wed, 19 Oct 2022 09:08:15 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Evgeniy Baskov <baskov@...ras.ru>
Cc:     Borislav Petkov <bp@...en8.de>, Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexey Khoroshilov <khoroshilov@...ras.ru>,
        lvc-project@...uxtesting.org, x86@...nel.org,
        linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH 04/16] x86/boot: Increase boot page table size

On Tue, 6 Sept 2022 at 12:41, Evgeniy Baskov <baskov@...ras.ru> wrote:
>
> Previous calculations ignored pages implicitly mapped by ACPI code,

I'm not sure I understand what this means. Which ACPI code and which
pages does it map?

> so theoretical upper limit is higher than was set.
>
> Using 4KB pages is desirable for better memory protection granularity.
> Approximately twice as much memory is required for those.
>
> Increase initial page table size to 64 4KB page tables.
>
> Signed-off-by: Evgeniy Baskov <baskov@...ras.ru>
> ---
>  arch/x86/include/asm/boot.h | 26 ++++++++++++++------------
>  1 file changed, 14 insertions(+), 12 deletions(-)
>
> diff --git a/arch/x86/include/asm/boot.h b/arch/x86/include/asm/boot.h
> index 9191280d9ea3..024d972c248e 100644
> --- a/arch/x86/include/asm/boot.h
> +++ b/arch/x86/include/asm/boot.h
> @@ -41,22 +41,24 @@
>  # define BOOT_STACK_SIZE       0x4000
>
>  # define BOOT_INIT_PGT_SIZE    (6*4096)
> -# ifdef CONFIG_RANDOMIZE_BASE
>  /*
>   * Assuming all cross the 512GB boundary:
>   * 1 page for level4
> - * (2+2)*4 pages for kernel, param, cmd_line, and randomized kernel
> - * 2 pages for first 2M (video RAM: CONFIG_X86_VERBOSE_BOOTUP).
> - * Total is 19 pages.
> + * (3+3)*2 pages for param and cmd_line
> + * (2+2+S)*2 pages for kernel and randomized kernel, where S is total number
> + *     of sections of kernel. Explanation: 2+2 are upper level page tables.
> + *     We can have only S unaligned parts of section: 1 at the end of the kernel
> + *     and (S-1) at the section borders. The start address of the kernel is
> + *     aligned, so an extra page table. There are at most S=6 sections in
> + *     vmlinux ELF image.
> + * 3 pages for first 2M (video RAM: CONFIG_X86_VERBOSE_BOOTUP).
> + * Total is 36 pages.
> + *
> + * Some pages are also required for UEFI memory map and
> + * ACPI table mappings, so we need to add extra space.
> + * FIXME: Figure out exact amount of pages.
>   */
> -#  ifdef CONFIG_X86_VERBOSE_BOOTUP
> -#   define BOOT_PGT_SIZE       (19*4096)
> -#  else /* !CONFIG_X86_VERBOSE_BOOTUP */
> -#   define BOOT_PGT_SIZE       (17*4096)
> -#  endif
> -# else /* !CONFIG_RANDOMIZE_BASE */
> -#  define BOOT_PGT_SIZE                BOOT_INIT_PGT_SIZE
> -# endif
> +# define BOOT_PGT_SIZE         (64*4096)
>
>  #else /* !CONFIG_X86_64 */
>  # define BOOT_STACK_SIZE       0x1000
> --
> 2.35.1
>