linux-kernel - Re: [PATCH v5 01/20] x86/efistub: Branch straight to kernel entry point from C code

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Message-ID: <20230607185301.GNZIDSDWX8xnTaeENe@fat_crate.local>
Date:   Wed, 7 Jun 2023 20:53:01 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
        Evgeniy Baskov <baskov@...ras.ru>,
        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>,
        Peter Jones <pjones@...hat.com>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Dave Young <dyoung@...hat.com>,
        Mario Limonciello <mario.limonciello@....com>,
        Kees Cook <keescook@...omium.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH v5 01/20] x86/efistub: Branch straight to kernel entry
 point from C code

On Wed, Jun 07, 2023 at 09:23:23AM +0200, Ard Biesheuvel wrote:
> -	return bzimage_addr;
> +	if (IS_ENABLED(CONFIG_X86_64))
> +		/* add offset of startup_64() */
> +		bzimage_addr += 0x200;

Uh, magic.

Well, there's this:

arch/x86/boot/compressed/head_64.S:

        .code64
        .org 0x200
SYM_CODE_START(startup_64)
        /*
         * 64bit entry is 0x200 and it is ABI so immutable!
         * We come here either from startup_32 or directly from a
         * 64bit bootloader.


Looking at Documentation/arch/x86/boot.rst, we actually say in the
xloadflags section:

  Bit 0 (read): XLF_KERNEL_64

        - If 1, this kernel has the legacy 64-bit entry point at 0x200.

and header.S sets that:

xloadflags:
#ifdef CONFIG_X86_64
# define XLF0 XLF_KERNEL_64                     /* 64-bit kernel */

so you checking CONFIG_X86_64 is probably ok.

It might be cleaner, though, if you test XLF_KERNEL_64 directly and act
accordingly, although, do I understand it correctly, that the EFI
libstub goes together with the kernel it was built for so the checks
would be doing the same thing...? I.e., the libstub cannot be somehow
"glued" with another kernel or so, which doesn't set CONFIG_X86_64.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette