linux-kernel - [tip: x86/urgent] x86/sev: Improve handling of writes to intercepted TSC MSRs

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Message-ID: <175499631833.1420.8242908162869935320.tip-bot2@tip-bot2>
Date: Tue, 12 Aug 2025 10:58:33 -0000
From: "tip-bot2 for Nikunj A Dadhania" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Sean Christopherson <seanjc@...gle.com>,
 Nikunj A Dadhania <nikunj@....com>, "Borislav Petkov (AMD)" <bp@...en8.de>,
 Tom Lendacky <thomas.lendacky@....com>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject:
 [tip: x86/urgent] x86/sev: Improve handling of writes to intercepted TSC MSRs

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     5eb1bcdb6a8c088514019c3a9bda5d565beed1af
Gitweb:        https://git.kernel.org/tip/5eb1bcdb6a8c088514019c3a9bda5d565beed1af
Author:        Nikunj A Dadhania <nikunj@....com>
AuthorDate:    Tue, 22 Jul 2025 13:18:53 +05:30
Committer:     Borislav Petkov (AMD) <bp@...en8.de>
CommitterDate: Tue, 12 Aug 2025 12:33:58 +02:00

x86/sev: Improve handling of writes to intercepted TSC MSRs

Currently, when a Secure TSC enabled SNP guest attempts to write to the
intercepted GUEST_TSC_FREQ MSR (a read-only MSR), the guest kernel response
incorrectly implies a VMM configuration error, when in fact it is the usual
VMM configuration to intercept writes to read-only MSRs, unless explicitly
documented.

Modify the intercepted TSC MSR #VC handling:
* Write to GUEST_TSC_FREQ will generate a #GP instead of terminating the
  guest
* Write to MSR_IA32_TSC will generate a #GP instead of silently ignoring it

However, continue to terminate the guest when reading from intercepted
GUEST_TSC_FREQ MSR with Secure TSC enabled, as intercepted reads indicate an
improper VMM configuration for Secure TSC enabled SNP guests.

  [ bp: simplify comment. ]

Fixes: 38cc6495cdec ("x86/sev: Prevent GUEST_TSC_FREQ MSR interception for Secure TSC enabled guests")
Suggested-by: Sean Christopherson <seanjc@...gle.com>
Signed-off-by: Nikunj A Dadhania <nikunj@....com>
Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
Reviewed-by: Tom Lendacky <thomas.lendacky@....com>
Link: https://lore.kernel.org/20250722074853.22253-1-nikunj@amd.com
---
 arch/x86/coco/sev/vc-handle.c | 31 ++++++++++++++++---------------
 1 file changed, 16 insertions(+), 15 deletions(-)

diff --git a/arch/x86/coco/sev/vc-handle.c b/arch/x86/coco/sev/vc-handle.c
index faf1fce..c3b4acb 100644
--- a/arch/x86/coco/sev/vc-handle.c
+++ b/arch/x86/coco/sev/vc-handle.c
@@ -371,29 +371,30 @@ static enum es_result __vc_handle_msr_caa(struct pt_regs *regs, bool write)
  * executing with Secure TSC enabled, so special handling is required for
  * accesses of MSR_IA32_TSC and MSR_AMD64_GUEST_TSC_FREQ.
  */
-static enum es_result __vc_handle_secure_tsc_msrs(struct pt_regs *regs, bool write)
+static enum es_result __vc_handle_secure_tsc_msrs(struct es_em_ctxt *ctxt, bool write)
 {
+	struct pt_regs *regs = ctxt->regs;
 	u64 tsc;
 
 	/*
-	 * GUEST_TSC_FREQ should not be intercepted when Secure TSC is enabled.
-	 * Terminate the SNP guest when the interception is enabled.
+	 * Writing to MSR_IA32_TSC can cause subsequent reads of the TSC to
+	 * return undefined values, and GUEST_TSC_FREQ is read-only. Generate
+	 * a #GP on all writes.
 	 */
-	if (regs->cx == MSR_AMD64_GUEST_TSC_FREQ)
-		return ES_VMM_ERROR;
+	if (write) {
+		ctxt->fi.vector = X86_TRAP_GP;
+		ctxt->fi.error_code = 0;
+		return ES_EXCEPTION;
+	}
 
 	/*
-	 * Writes: Writing to MSR_IA32_TSC can cause subsequent reads of the TSC
-	 *         to return undefined values, so ignore all writes.
-	 *
-	 * Reads: Reads of MSR_IA32_TSC should return the current TSC value, use
-	 *        the value returned by rdtsc_ordered().
+	 * GUEST_TSC_FREQ read should not be intercepted when Secure TSC is
+	 * enabled. Terminate the guest if a read is attempted.
 	 */
-	if (write) {
-		WARN_ONCE(1, "TSC MSR writes are verboten!\n");
-		return ES_OK;
-	}
+	if (regs->cx == MSR_AMD64_GUEST_TSC_FREQ)
+		return ES_VMM_ERROR;
 
+	/* Reads of MSR_IA32_TSC should return the current TSC value. */
 	tsc = rdtsc_ordered();
 	regs->ax = lower_32_bits(tsc);
 	regs->dx = upper_32_bits(tsc);
@@ -416,7 +417,7 @@ static enum es_result vc_handle_msr(struct ghcb *ghcb, struct es_em_ctxt *ctxt)
 	case MSR_IA32_TSC:
 	case MSR_AMD64_GUEST_TSC_FREQ:
 		if (sev_status & MSR_AMD64_SNP_SECURE_TSC)
-			return __vc_handle_secure_tsc_msrs(regs, write);
+			return __vc_handle_secure_tsc_msrs(ctxt, write);
 		break;
 	default:
 		break;