linux-kernel - Re: [RFC PATCH 4/9] x86/purgatory: Avoid absolute reference to GDT

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Date: Wed, 24 Apr 2024 13:38:59 -0400
From: Brian Gerst <brgerst@...il.com>
To: Ard Biesheuvel <ardb+git@...gle.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, 
	Ard Biesheuvel <ardb@...nel.org>, Arnd Bergmann <arnd@...db.de>, Eric Biederman <ebiederm@...ssion.com>, 
	kexec@...ts.infradead.org, Nathan Chancellor <nathan@...nel.org>, 
	Nick Desaulniers <ndesaulniers@...gle.com>, Kees Cook <keescook@...omium.org>, 
	Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>, 
	Masahiro Yamada <masahiroy@...nel.org>
Subject: Re: [RFC PATCH 4/9] x86/purgatory: Avoid absolute reference to GDT

On Wed, Apr 24, 2024 at 12:06 PM Ard Biesheuvel <ardb+git@...gle.com> wrote:
>
> From: Ard Biesheuvel <ardb@...nel.org>
>
> The purgatory is almost entirely position independent, without any need
> for any relocation processing at load time except for the reference to
> the GDT in the entry code. Generate this reference at runtime instead,
> to remove the last R_X86_64_64 relocation from this code.
>
> While the GDT itself needs to be preserved in memory as long as it is
> live, the GDT descriptor that is used to program the GDT can be
> discarded so it can be allocated on the stack.
>
> Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> ---
>  arch/x86/purgatory/entry64.S | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/purgatory/entry64.S b/arch/x86/purgatory/entry64.S
> index 9913877b0dbe..888661d9db9c 100644
> --- a/arch/x86/purgatory/entry64.S
> +++ b/arch/x86/purgatory/entry64.S
> @@ -16,7 +16,11 @@
>
>  SYM_CODE_START(entry64)
>         /* Setup a gdt that should be preserved */
> -       lgdt gdt(%rip)
> +       leaq    gdt(%rip), %rax
> +       pushq   %rax
> +       pushw   $gdt_end - gdt - 1
> +       lgdt    (%rsp)
> +       addq    $10, %rsp

This misaligns the stack, pushing 16 bytes on the stack but only
removing 10 (decimal).

>
>         /* load the data segments */
>         movl    $0x18, %eax     /* data segment */
> @@ -83,8 +87,8 @@ SYM_DATA_START_LOCAL(gdt)
>          * 0x08 unused
>          * so use them as gdt ptr

obsolete comment

>          */
> -       .word gdt_end - gdt - 1
> -       .quad gdt
> +       .word 0
> +       .quad 0
>         .word 0, 0, 0

This can be condensed down to:
        .quad 0, 0

>
>         /* 0x10 4GB flat code segment */
> --
> 2.44.0.769.g3c40516874-goog

Brian Gerst