linux-kernel - Re: [PATCH v6 19/22] x86/boot: Create a confined code area for startup code

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Message-ID: <20250811101955.GOaJnDy0-uy0quuouD@fat_crate.local>
Date: Mon, 11 Aug 2025 12:19:55 +0200
From: Borislav Petkov <bp@...en8.de>
To: Ard Biesheuvel <ardb+git@...gle.com>
Cc: linux-kernel@...r.kernel.org, linux-efi@...r.kernel.org, x86@...nel.org,
	Ard Biesheuvel <ardb@...nel.org>, Ingo Molnar <mingo@...nel.org>,
	Kevin Loughlin <kevinloughlin@...gle.com>,
	Tom Lendacky <thomas.lendacky@....com>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Nikunj A Dadhania <nikunj@....com>
Subject: Re: [PATCH v6 19/22] x86/boot: Create a confined code area for
 startup code

On Tue, Jul 22, 2025 at 09:27:28AM +0200, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@...nel.org>
> 
> In order to be able to have tight control over which code may execute
> from the early 1:1 mapping of memory, but still link vmlinux as a single
> executable, prefix all symbol references in startup code with __pi_, and
> invoke it from outside using the __pi_ prefix.
> 
> Use objtool to check that no absolute symbol references are present in
> the startup code, as these cannot be used from code running from the 1:1
> mapping.
> 
> Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> ---
>  arch/x86/boot/startup/Makefile     | 14 ++++++++++++++
>  arch/x86/boot/startup/sev-shared.c |  4 +---
>  arch/x86/boot/startup/sme.c        |  1 -
>  arch/x86/coco/sev/core.c           |  2 +-
>  arch/x86/include/asm/setup.h       |  1 +
>  arch/x86/include/asm/sev.h         |  1 +
>  arch/x86/kernel/head64.c           |  2 +-
>  arch/x86/kernel/head_64.S          |  8 ++++----
>  arch/x86/mm/mem_encrypt_boot.S     |  6 +++---
>  tools/objtool/check.c              |  3 ++-
>  10 files changed, 28 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/x86/boot/startup/Makefile b/arch/x86/boot/startup/Makefile
> index 32737f4ab5a8..e8fdf020b422 100644
> --- a/arch/x86/boot/startup/Makefile
> +++ b/arch/x86/boot/startup/Makefile
> @@ -4,6 +4,7 @@ KBUILD_AFLAGS		+= -D__DISABLE_EXPORTS
>  KBUILD_CFLAGS		+= -D__DISABLE_EXPORTS -mcmodel=small -fPIC \
>  			   -Os -DDISABLE_BRANCH_PROFILING \
>  			   $(DISABLE_STACKLEAK_PLUGIN) \
> +			   $(DISABLE_LATENT_ENTROPY_PLUGIN) \
>  			   -fno-stack-protector -D__NO_FORTIFY \
>  			   -fno-jump-tables \
>  			   -include $(srctree)/include/linux/hidden.h

Commit message should say something about that hunk I guess. It looks like it
disables the instrumentation that this plugin adds but I dunno.

-- 
Regards/Gruss,
    Boris.

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