linux-kernel - Re: [PATCH v2 00/15] x86/boot: Rework PE header generation

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Date:   Fri, 15 Sep 2023 11:22:27 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Ard Biesheuvel <ardb@...gle.com>
Cc:     linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ard Biesheuvel <ardb@...nel.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 v2 00/15] x86/boot: Rework PE header generation


* Ard Biesheuvel <ardb@...gle.com> wrote:

> From: Ard Biesheuvel <ardb@...nel.org>
> 
> Now that the EFI stub boot flow no longer relies on memory that is
> executable and writable at the same time, we can reorganize the PE/COFF
> view of the kernel image and expose the decompressor binary's code and
> r/o data as a .text section and data/bss as a .data section, using 4k
> alignment and limited permissions.
> 
> Doing so is necessary for compatibility with hardening measures that are
> being rolled out on x86 PCs built to run Windows (i.e., the majority of
> them). The EFI boot environment that the Linux EFI stub executes in is
> especially sensitive to safety issues, given that a vulnerability in the
> loader of one OS can be abused to attack another.
> 
> In true x86 fashion, this is a lot more complicated than on other
> architectures, which have implemented this code/data split with 4k
> alignment from the beginning. The complicating factor here is that the
> boot image consists of two different parts, which are stitched together
> and fixed up using a special build tool.
> 
> After this series is applied, the only remaining task performed by the
> build tool is generating the CRC-32. Even though this checksum is
> usually wrong (given that distro kernels are signed for secure boot in a
> way that corrupts the CRC), this feature is retained as we cannot be
> sure that nobody is relying on this.
> 
> This supersedes the work proposed by Evgeniy last year, which did a
> major rewrite of the build tool in order to clean it up, before updating
> it to generate the new 4k aligned image layout. As this series proves,
> the build tool is mostly unnecessary, and we have too many of those
> already.
> 
> Changes since v1:
> - drop patch that removed the CRC and the build tool
> - do not use fixed setup_size but derive it in the setup.ld linker
>   script
> - reorganize the PE header so the .compat section only covers its
>   payload and the padding that follows it
> - add hpa's ack to patch #4
> 
> Cc: Evgeniy Baskov <baskov@...ras.ru>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Peter Jones <pjones@...hat.com>
> Cc: Matthew Garrett <mjg59@...f.ucam.org>
> Cc: Gerd Hoffmann <kraxel@...hat.com>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> 
> Ard Biesheuvel (15):
>   x86/efi: Drop EFI stub .bss from .data section
>   x86/efi: Disregard setup header of loaded image
>   x86/efi: Drop alignment flags from PE section headers
>   x86/boot: Remove the 'bugger off' message
>   x86/boot: Omit compression buffer from PE/COFF image memory footprint
>   x86/boot: Drop redundant code setting the root device
>   x86/boot: Grab kernel_info offset from zoffset header directly
>   x86/boot: Drop references to startup_64

I've applied these first 8 patches to tip:x86/boot with minor edits.
(Please preserve existing comment capitalization conventions ...)

>   x86/boot: Set EFI handover offset directly in header asm
>   x86/boot: Define setup size in linker script
>   x86/boot: Derive file size from _edata symbol
>   x86/boot: Construct PE/COFF .text section from assembler
>   x86/boot: Drop PE/COFF .reloc section
>   x86/boot: Split off PE/COFF .data section
>   x86/boot: Increase section and file alignment to 4k/512

The rest conflicted with recent upstream changes, and I suppose it's
prudent to test these changes bit by bit anyway.

Thanks,

	Ingo