Re: [PATCH v3 12/75] x86/boot/compressed/64: Switch to __KERNEL_CS after GDT is loaded

[Borislav Petkov]

On Tue, Apr 28, 2020 at 05:16:22PM +0200, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@xxxxxxx>
> 
> When the pre-decompression code loads its first GDT in startup_64, it is
> still running on the CS value of the previous GDT. In the case of SEV-ES
> this is the EFI GDT.
> 
> To make exception handling work (especially IRET) the CPU needs to
> switch to a CS value in the current GDT, so jump to __KERNEL_CS after
> the first GDT is loaded.
> 
> Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>
> ---
>  arch/x86/boot/compressed/head_64.S | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> index 4f7e6b84be07..6b11060c3a0f 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -393,6 +393,14 @@ SYM_CODE_START(startup_64)
>  	addq	%rax, 2(%rax)
>  	lgdt	(%rax)
>  
> +	/* Reload CS so IRET returns to a CS actually in the GDT */
> +	pushq	$__KERNEL_CS
> +	leaq	.Lon_kernel_cs(%rip), %rax
> +	pushq	%rax
> +	lretq
> +
> +.Lon_kernel_cs:
> +
>  	/*
>  	 * paging_prepare() sets up the trampoline and checks if we need to
>  	 * enable 5-level paging.
> -- 

So I'm thinking I should take this one even now on the grounds that
it sanitizes CS to something known-good than what was there before and
who knows what set it and loaded the kernel...?

And that is a good thing in itself.

-- 
Regards/Gruss,
    Boris.

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