[PATCH 3/3] KVM: x86: Forbid VMM to set SYNIC/STIMER MSRs when SynIC wasn't activated

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Setting non-zero values to SYNIC/STIMER MSRs activates certain features,
this should not happen when KVM_CAP_HYPERV_SYNIC{,2} was not activated.

Note, it would've been better to forbid writing anything to SYNIC/STIMER
MSRs, including zeroes, however, at least QEMU tries clearing
HV_X64_MSR_STIMER0_CONFIG without SynIC. HV_X64_MSR_EOM MSR is somewhat
'special' as writing zero there triggers an action, this also should not
happen when SynIC wasn't activated.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
 arch/x86/kvm/hyperv.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index a79a094c1266..123b677111c5 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -243,7 +243,7 @@ static int synic_set_msr(struct kvm_vcpu_hv_synic *synic,
 	struct kvm_vcpu *vcpu = hv_synic_to_vcpu(synic);
 	int ret;
 
-	if (!synic->active && !host)
+	if (!synic->active && (!host || data))
 		return 1;
 
 	trace_kvm_hv_synic_set_msr(vcpu->vcpu_id, msr, data, host);
@@ -289,6 +289,9 @@ static int synic_set_msr(struct kvm_vcpu_hv_synic *synic,
 	case HV_X64_MSR_EOM: {
 		int i;
 
+		if (!synic->active)
+			break;
+
 		for (i = 0; i < ARRAY_SIZE(synic->sint); i++)
 			kvm_hv_notify_acked_sint(vcpu, i);
 		break;
@@ -668,7 +671,7 @@ static int stimer_set_config(struct kvm_vcpu_hv_stimer *stimer, u64 config,
 	struct kvm_vcpu_hv *hv_vcpu = to_hv_vcpu(vcpu);
 	struct kvm_vcpu_hv_synic *synic = to_hv_synic(vcpu);
 
-	if (!synic->active && !host)
+	if (!synic->active && (!host || config))
 		return 1;
 
 	if (unlikely(!host && hv_vcpu->enforce_cpuid && new_config.direct_mode &&
@@ -697,7 +700,7 @@ static int stimer_set_count(struct kvm_vcpu_hv_stimer *stimer, u64 count,
 	struct kvm_vcpu *vcpu = hv_stimer_to_vcpu(stimer);
 	struct kvm_vcpu_hv_synic *synic = to_hv_synic(vcpu);
 
-	if (!synic->active && !host)
+	if (!synic->active && (!host || count))
 		return 1;
 
 	trace_kvm_hv_stimer_set_count(hv_stimer_to_vcpu(stimer)->vcpu_id,
-- 
2.35.1