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

[Borislav Petkov]

On Wed, Dec 18, 2024 at 10:50:07AM +0530, Nikunj A. Dadhania wrote:
> With the condition inside the function, even tough the MSR is not
> valid in this configuration, I am getting value 0. Is this behavior
> acceptable ?

The whole untested diff, should DTRT this time:

diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
index 10980898c054..96a9ee93f9cb 100644
--- a/arch/x86/coco/sev/core.c
+++ b/arch/x86/coco/sev/core.c
@@ -1441,10 +1441,25 @@ static enum es_result __vc_handle_msr_caa(struct pt_regs *regs, bool write)
  */
 static enum es_result __vc_handle_msr_tsc(struct pt_regs *regs, bool write)
 {
+	bool sec_tsc = sev_status & MSR_AMD64_SNP_SECURE_TSC;
 	u64 tsc;
 
-	if (write)
-		return ES_OK;
+	/*
+	 * The GUEST_TSC_FREQ MSR should not be intercepted when secure
+	 * TSC is enabled so terminate the guest. For non-secure TSC
+	 * guests, that MSR is #GP(0).
+	 */
+	if (regs->cx == MSR_AMD64_GUEST_TSC_FREQ) {
+		if (sec_tsc)
+			return ES_VMM_ERROR;
+		else
+			return ES_UNSUPPORTED;
+	}
+
+	if (write) {
+		WARN_ONCE(1, "TSC MSR writes are verboten!\n");
+		return ES_UNSUPPORTED;
+	}
 
 	tsc = rdtsc_ordered();
 	regs->ax = lower_32_bits(tsc);
@@ -1462,19 +1477,15 @@ static enum es_result vc_handle_msr(struct ghcb *ghcb, struct es_em_ctxt *ctxt)
 	/* Is it a WRMSR? */
 	write = ctxt->insn.opcode.bytes[1] == 0x30;
 
-	if (regs->cx == MSR_SVSM_CAA)
+	switch(regs->cx) {
+	case MSR_SVSM_CAA:
 		return __vc_handle_msr_caa(regs, write);
-
-	if (regs->cx == MSR_IA32_TSC && (sev_status & MSR_AMD64_SNP_SECURE_TSC))
+	case MSR_IA32_TSC:
+	case MSR_AMD64_GUEST_TSC_FREQ:
 		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;
-
+	default:
+		break;
+	}
 
 	ghcb_set_rcx(ghcb, regs->cx);
 	if (write) {

-- 
Regards/Gruss,
    Boris.

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