linux-kernel - Re: [PATCH 17/17] x86/boot: Drop CRC-32 checksum and the build tool that generates it

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Date:   Sun, 20 Aug 2023 14:57:40 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     "H. Peter Anvin" <hpa@...or.com>
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>,
        Marvin Häuser <mhaeuser@...teo.de>
Subject: Re: [PATCH 17/17] x86/boot: Drop CRC-32 checksum and the build tool
 that generates it

On Sun, 20 Aug 2023 at 03:03, H. Peter Anvin <hpa@...or.com> wrote:
>
>
>
> On 8/18/23 06:44, Ard Biesheuvel wrote:
> > The only remaining task carried out by the boot/tools/build.c build tool
> > is generating the CRC-32 checksum of the bzImage. This feature was added
> > in commit
> >
> >    7d6e737c8d2698b6 ("x86: add a crc32 checksum to the kernel image.")
> >
> > without any motivation (or any commit log text, for that matter). This
> > checksum is not verified by any known bootloader, and given that
> >
> > a) the checksum of the entire bzImage is reported by most tools (zlib,
> >     rhash) as 0xffffffff and not 0x0 as documented,
> > b) the checksum is corrupted when the image is signed for secure boot,
> >     which means that no distro ships x86 images with valid CRCs,
> >
> > it seems quite unlikely that this checksum is being used, so let's just
> > drop it, along with the tool that generates it.
> >
>
> This one I have concerns with.
>

I understand. I deliberately put this change at the very end because I
was anticipating some debate on this.

Do you have any recollection of why this CRC32 was introduced in the
first place? The commit logs are empty and the lore thread doesn't
contain any justification either.