linux-kernel - Re: [PATCH 15/17] x86/boot: Split off PE/COFF .data section

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Message-Id: <98D635CA-43D4-4A0B-8791-2FDC1DC695B3@posteo.de>
Date:   Fri, 18 Aug 2023 14:35:01 +0000
From:   Marvin Häuser <mhaeuser@...teo.de>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
        Evgeniy Baskov <baskov@...ras.ru>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Jones <pjones@...hat.com>,
        Matthew Garrett <mjg59@...f.ucam.org>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 15/17] x86/boot: Split off PE/COFF .data section

Hi Ard,

Thanks for your effort! Not sure what the documentation policies are, but might it be worth mentioning that we cannot have .rdata at this time, because current-gen EFI will map it as RW?

Best regards,
Marvin

> On Aug 18, 2023, at 15:45, Ard Biesheuvel <ardb@...nel.org> wrote:
> Describe the code and data of the decompressor binary using separate
> .text and .data PE/COFF sections, so that we will be able to map them
> using restricted permissions once we increase the section and file
> alignment sufficiently. This avoids the need for memory mappings that
> are writable and executable at the same time, which is something that
> is best avoided for security reasons.
> 
> Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> ---
> arch/x86/boot/Makefile |  2 +-
> arch/x86/boot/header.S | 19 +++++++++++++++----
> 2 files changed, 16 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/boot/Makefile b/arch/x86/boot/Makefile
> index b26e30a2d865f72d..50c50fce646e2417 100644
> --- a/arch/x86/boot/Makefile
> +++ b/arch/x86/boot/Makefile
> @@ -90,7 +90,7 @@ $(obj)/vmlinux.bin: $(obj)/compressed/vmlinux FORCE
> 
> SETUP_OBJS = $(addprefix $(obj)/,$(setup-y))
> 
> -sed-zoffset := -e 's/^\([0-9a-fA-F]*\) [a-zA-Z] \(startup_32\|efi.._stub_entry\|efi\(32\)\?_pe_entry\|input_data\|kernel_info\|_end\|_ehead\|_text\|_edata\|z_.*\)$$/\#define ZO_\2 0x\1/p'
> +sed-zoffset := -e 's/^\([0-9a-fA-F]*\) [a-zA-Z] \(startup_32\|efi.._stub_entry\|efi\(32\)\?_pe_entry\|input_data\|kernel_info\|_end\|_ehead\|_text\|_e\?data\|z_.*\)$$/\#define ZO_\2 0x\1/p'
> 
> quiet_cmd_zoffset = ZOFFSET $@
>       cmd_zoffset = $(NM) $< | sed -n $(sed-zoffset) > $@
> diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
> index ccfb7a7d8c29275e..25dda40dacb52292 100644
> --- a/arch/x86/boot/header.S
> +++ b/arch/x86/boot/header.S
> @@ -79,9 +79,9 @@ optional_header:
>    .byte    0x02                # MajorLinkerVersion
>    .byte    0x14                # MinorLinkerVersion
> 
> -    .long    setup_size + ZO__end - 0x200    # SizeOfCode
> +    .long    ZO__data            # SizeOfCode
> 
> -    .long    0                # SizeOfInitializedData
> +    .long    ZO__end - ZO__data        # SizeOfInitializedData
>    .long    0                # SizeOfUninitializedData
> 
>    .long    setup_size + ZO_efi_pe_entry    # AddressOfEntryPoint
> @@ -182,9 +182,9 @@ section_table:
>    .byte    0
>    .byte    0
>    .byte    0
> -    .long    ZO__end
> +    .long    ZO__data
>    .long    setup_size
> -    .long    ZO__edata            # Size of initialized data
> +    .long    ZO__data            # Size of initialized data
>                        # on disk
>    .long    setup_size
>    .long    0                # PointerToRelocations
> @@ -195,6 +195,17 @@ section_table:
>        IMAGE_SCN_MEM_READ        | \
>        IMAGE_SCN_MEM_EXECUTE        # Characteristics
> 
> +    .ascii    ".data\0\0\0"
> +    .long    ZO__end - ZO__data        # VirtualSize
> +    .long    setup_size + ZO__data        # VirtualAddress
> +    .long    ZO__edata - ZO__data        # SizeOfRawData
> +    .long    setup_size + ZO__data        # PointerToRawData
> +
> +    .long    0, 0, 0
> +    .long    IMAGE_SCN_CNT_INITIALIZED_DATA    | \
> +        IMAGE_SCN_MEM_READ        | \
> +        IMAGE_SCN_MEM_WRITE        # Characteristics
> +
>    .set    section_count, (. - section_table) / 40
> #endif /* CONFIG_EFI_STUB */
> 
> -- 
> 2.39.2