linux-kernel - Re: [PATCH v15 06/13] x86/sev: Prevent GUEST_TSC_FREQ MSR interception for Secure TSC enabled guests

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Message-ID: <20241210121127.GBZ1gv74Q6IRtAS1pl@fat_crate.local>
Date: Tue, 10 Dec 2024 13:11:27 +0100
From: Borislav Petkov <bp@...en8.de>
To: Nikunj A Dadhania <nikunj@....com>
Cc: linux-kernel@...r.kernel.org, thomas.lendacky@....com, x86@...nel.org,
	kvm@...r.kernel.org, mingo@...hat.com, tglx@...utronix.de,
	dave.hansen@...ux.intel.com, pgonda@...gle.com, seanjc@...gle.com,
	pbonzini@...hat.com
Subject: Re: [PATCH v15 06/13] x86/sev: Prevent GUEST_TSC_FREQ MSR
 interception for Secure TSC enabled guests

On Tue, Dec 03, 2024 at 02:30:38PM +0530, Nikunj A Dadhania wrote:
> diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
> index af28fb962309..59c5e716fdd1 100644
> --- a/arch/x86/coco/sev/core.c
> +++ b/arch/x86/coco/sev/core.c
> @@ -1473,6 +1473,14 @@ static enum es_result vc_handle_msr(struct ghcb *ghcb, struct es_em_ctxt *ctxt)
>  	if (regs->cx == MSR_IA32_TSC && (sev_status & MSR_AMD64_SNP_SECURE_TSC))
>  		return __vc_handle_msr_tsc(regs, write);
>  
> +	/*
> +	 * GUEST_TSC_FREQ should not be intercepted when Secure TSC is
> +	 * enabled. Terminate the SNP guest when the interception is enabled.
> +	 */
> +	if (regs->cx == MSR_AMD64_GUEST_TSC_FREQ && (sev_status & MSR_AMD64_SNP_SECURE_TSC))
> +		return ES_VMM_ERROR;
> +
> +

If you merge this logic into the switch-case, the patch becomes even easier
and the code cleaner:

diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
index 050170eb28e6..35d9a3bb4b06 100644
--- a/arch/x86/coco/sev/core.c
+++ b/arch/x86/coco/sev/core.c
@@ -1446,6 +1446,13 @@ static enum es_result __vc_handle_msr_tsc(struct pt_regs *regs, bool write)
 	if (!(sev_status & MSR_AMD64_SNP_SECURE_TSC))
 		goto read_tsc;
 
+	/*
+	 * GUEST_TSC_FREQ should not be intercepted when Secure TSC is
+	 * enabled. Terminate the SNP guest when the interception is enabled.
+	 */
+	if (regs->cx == MSR_AMD64_GUEST_TSC_FREQ)
+		return ES_VMM_ERROR;
+
 	if (write) {
 		WARN_ONCE(1, "TSC MSR writes are verboten!\n");
 		return ES_UNSUPPORTED;
@@ -1472,6 +1479,7 @@ static enum es_result vc_handle_msr(struct ghcb *ghcb, struct es_em_ctxt *ctxt)
 	case MSR_SVSM_CAA:
 		return __vc_handle_msr_caa(regs, write);
 	case MSR_IA32_TSC:
+	case MSR_AMD64_GUEST_TSC_FREQ:
 		return __vc_handle_msr_tsc(regs, write);
 	default:
 		break;

-- 
Regards/Gruss,
    Boris.

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