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

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>

diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index f715b5a2b0e422cd8c25448b8421569fbcbe4714..4177c17a26bf71fa7eac9e6817a9974796be6028 100644 (file)
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -239,7 +239,7 @@ 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);
@@ -285,6 +285,9 @@ 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;
@@ -664,7 +667,7 @@ 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 &&
@@ -693,7 +696,7 @@ 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,