linux-kernel - Re: [PATCH 4/7] x86/boot/compressed/64: Add 32-bit boot #VC handler

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Date:   Thu, 25 Feb 2021 13:13:08 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Joerg Roedel <joro@...tes.org>
Cc:     x86@...nel.org, Joerg Roedel <jroedel@...e.de>, hpa@...or.com,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        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>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Martin Radev <martin.b.radev@...il.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 4/7] x86/boot/compressed/64: Add 32-bit boot #VC handler

On Wed, Feb 10, 2021 at 11:21:32AM +0100, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@...e.de>
> 
> Add a #VC exception handler which is used when the kernel still executes
> in protected mode. This boot-path already uses CPUID, which will cause #VC
> exceptions in an SEV-ES guest.
> 
> Signed-off-by: Joerg Roedel <jroedel@...e.de>
> ---
>  arch/x86/boot/compressed/head_64.S     |  6 ++
>  arch/x86/boot/compressed/mem_encrypt.S | 77 +++++++++++++++++++++++++-
>  2 files changed, 82 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> index 8deeec78cdb4..eadaa0a082b8 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -34,6 +34,7 @@
>  #include <asm/asm-offsets.h>
>  #include <asm/bootparam.h>
>  #include <asm/desc_defs.h>
> +#include <asm/trapnr.h>
>  #include "pgtable.h"
>  
>  /*
> @@ -856,6 +857,11 @@ SYM_FUNC_START(startup32_set_idt_entry)
>  SYM_FUNC_END(startup32_set_idt_entry)
>  
>  SYM_FUNC_START(startup32_load_idt)
> +	/* #VC handler */
> +	leal    rva(startup32_vc_handler)(%ebp), %eax
> +	movl    $X86_TRAP_VC, %edx
> +	call    startup32_set_idt_entry
> +
>  	/* Load IDT */
>  	leal	rva(boot32_idt)(%ebp), %eax
>  	movl	%eax, rva(boot32_idt_desc+2)(%ebp)
> diff --git a/arch/x86/boot/compressed/mem_encrypt.S b/arch/x86/boot/compressed/mem_encrypt.S
> index aa561795efd1..350ecb56c7e4 100644
> --- a/arch/x86/boot/compressed/mem_encrypt.S
> +++ b/arch/x86/boot/compressed/mem_encrypt.S
> @@ -67,10 +67,85 @@ SYM_FUNC_START(get_sev_encryption_bit)
>  	ret
>  SYM_FUNC_END(get_sev_encryption_bit)
>  
> +/*
> + * Emit code to request an CPUID register from the Hypervisor using
> + * the MSR-based protocol.
> + *
> + * fn: The register containing the CPUID function
> + * reg: Register requested
> + *	1 = EAX
> + *	2 = EBX
> + *	3 = ECX
> + *	4 = EDX
> + *
> + * Result is in EDX. Jumps to .Lfail on error
> + */
> +.macro	SEV_ES_REQ_CPUID fn:req reg:req

I'm wondering - instead of replicating this 4 times, can this be a
function which you CALL? You do have a stack so you should be able to.

> +	/* Request CPUID[%ebx].EAX */
> +	movl	$\reg, %eax
> +	shll	$30, %eax
> +	orl	$0x00000004, %eax
> +	movl	\fn, %edx
> +	movl	$MSR_AMD64_SEV_ES_GHCB, %ecx
> +	wrmsr
> +	rep; vmmcall
> +	rdmsr
> +	/* Check response code */

Before you do that, I guess you wanna check:

GHCBData[29:12] – Reserved, must be zero

in the HV response.

Thx.

-- 
Regards/Gruss,
    Boris.

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