[PATCH v4 4/5] KVM: SVM: Prevent writes to TSC MSR when Secure TSC is enabled

Nikunj A Dadhania
2025-03-10T06:45:53Z

Disallow writes to MSR_IA32_TSC for Secure TSC enabled SNP guests. Even if
KVM attempts to emulate such writes, TSC calculation will ignore the
TSC_SCALE and TSC_OFFSET present in the VMCB. Instead, it will use
GUEST_TSC_SCALE and GUEST_TSC_OFFSET stored in the VMSA.

Additionally, incorporate a check for protected guest state to allow the
VMM to initialize the TSC MSR.

Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
---
arch/x86/kvm/svm/svm.c | 19 +++++++++++++++++++
1 file changed, 19 insertions(+)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index e65721db1f81..1652848b0240 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3161,6 +3161,25 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)

svm->tsc_aux = data;
break;
+ case MSR_IA32_TSC:
+ /*
+ * For Secure TSC enabled VM, do not emulate TSC write as the
+ * TSC calculation ignores the TSC_OFFSET and TSC_SCALE control
+ * fields.
+ *
+ * Guest writes: Record the error and return a #GP.
+ * Host writes are ignored.
+ */
+ if (snp_secure_tsc_enabled(vcpu->kvm)) {
+ if (!msr->host_initiated) {
+ vcpu_unimpl(vcpu, "unimplemented IA32_TSC for Secure TSC\n");
+ return 1;
+ } else
+ return 0;
+ }
+
+ ret = kvm_set_msr_common(vcpu, msr);
+ break;
case MSR_IA32_DEBUGCTLMSR:
if (!lbrv) {
kvm_pr_unimpl_wrmsr(vcpu, ecx, data);
--
2.43.0