[PATCH v2] x86/sev: Improve handling of writes to intercepted TSC MSRs

[Nikunj A Dadhania]

From: Sean Christopherson <seanjc@xxxxxxxxxx>

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

Add a WARN_ONCE to log the incident, as well-behaved SNP guest kernels
should never attempt to write to these MSRs.

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.

Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Co-developed-by: Nikunj A Dadhania <nikunj@xxxxxxx>
Signed-off-by: Nikunj A Dadhania <nikunj@xxxxxxx>
---
 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 faf1fce89ed4..18be9f8bd015 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 pt_regs *regs, bool write,
+						  struct es_em_ctxt  *ctxt)
 {
 	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, but WARN to log a kernel bug.
 	 */
-	if (regs->cx == MSR_AMD64_GUEST_TSC_FREQ)
-		return ES_VMM_ERROR;
+	if (WARN_ON_ONCE(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 SNP guest when the interception is enabled.
 	 */
-	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(regs, write, ctxt);
 		break;
 	default:
 		break;

base-commit: 9f4a425223f3bd8ccaebc7f4f42b1d8c5f12fb45
-- 
2.43.0