[PATCH 5.17 0998/1126] KVM: x86: Forbid VMM to set SYNIC/STIMER MSRs when SynIC wasnt activated

Greg Kroah-Hartman
2022-04-05T10:46:32Z

From: Vitaly Kuznetsov <vkuznets@redhat.com>

commit b1e34d325397a33d97d845e312d7cf2a8b646b44 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
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
@@ -236,7 +236,7 @@ static int synic_set_msr(struct kvm_vcpu
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);
@@ -282,6 +282,9 @@ static int synic_set_msr(struct kvm_vcpu
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;
@@ -661,7 +664,7 @@ static int stimer_set_config(struct kvm_
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 &&
@@ -690,7 +693,7 @@ static int stimer_set_count(struct kvm_v
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,