From: Vitaly Kuznetsov <vkuznets@redhat.com>
Date: Fri, 25 Mar 2022 14:21:40 +0100
Subject: KVM: x86: Forbid VMM to set SYNIC/STIMER MSRs when SynIC wasn't
 activated
Git-commit: b1e34d325397a33d97d845e312d7cf2a8b646b44
Patch-mainline: v5.18-rc1
References: bsc#1200788 CVE-2022-2153

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>
Message-Id: <20220325132140.25650-4-vkuznets@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Dario Faggioli <dfaggioli@suse.com>
---
 arch/x86/kvm/hyperv.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -205,7 +205,7 @@
 	struct kvm_vcpu *vcpu = 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);
@@ -251,6 +251,9 @@
 	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;
@@ -520,7 +523,7 @@
 	struct kvm_vcpu *vcpu = stimer_to_vcpu(stimer);
 	struct kvm_vcpu_hv_synic *synic = vcpu_to_synic(vcpu);
 
-	if (!synic->active && !host)
+	if (!synic->active && (!host || config))
 		return 1;
 
 	trace_kvm_hv_stimer_set_config(stimer_to_vcpu(stimer)->vcpu_id,
@@ -544,7 +547,7 @@
 	struct kvm_vcpu *vcpu = stimer_to_vcpu(stimer);
 	struct kvm_vcpu_hv_synic *synic = vcpu_to_synic(vcpu);
 
-	if (!synic->active && !host)
+	if (!synic->active && (!host || count))
 		return 1;
 
 	trace_kvm_hv_stimer_set_count(stimer_to_vcpu(stimer)->vcpu_id,