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

Borislav Petkov
2020-05-04T10:41:39Z

On Tue, Apr 28, 2020 at 05:16:22PM +0200, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@suse.de>
>
> 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@suse.de>
> ---
> 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.