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Message-ID: <20200504104129.GD15046@zn.tnic>
Date: Mon, 4 May 2020 12:41:29 +0200
From: Borislav Petkov <bp@...en8.de>
To: Joerg Roedel <joro@...tes.org>
Cc: x86@...nel.org, hpa@...or.com, Andy Lutomirski <luto@...nel.org>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Peter Zijlstra <peterz@...radead.org>,
Thomas Hellstrom <thellstrom@...are.com>,
Jiri Slaby <jslaby@...e.cz>,
Dan Williams <dan.j.williams@...el.com>,
Tom Lendacky <thomas.lendacky@....com>,
Juergen Gross <jgross@...e.com>,
Kees Cook <keescook@...omium.org>,
David Rientjes <rientjes@...gle.com>,
Cfir Cohen <cfir@...gle.com>,
Erdem Aktas <erdemaktas@...gle.com>,
Masami Hiramatsu <mhiramat@...nel.org>,
Mike Stunes <mstunes@...are.com>,
Joerg Roedel <jroedel@...e.de>, linux-kernel@...r.kernel.org,
kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v3 12/75] x86/boot/compressed/64: Switch to __KERNEL_CS
after GDT is loaded
On Tue, Apr 28, 2020 at 05:16:22PM +0200, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@...e.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@...e.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.
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