linux-kernel - [PATCH 5.4] KVM: x86: Forbid VMM to set SYNIC/STIMER MSRs when SynIC wasn't activated

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Date:   Mon,  4 Apr 2022 11:43:52 -0400
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     stable@...r.kernel.org, Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: [PATCH 5.4] KVM: x86: Forbid VMM to set SYNIC/STIMER MSRs when SynIC wasn't activated

From: Vitaly Kuznetsov <vkuznets@...hat.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@...hat.com>
Message-Id: <20220325132140.25650-4-vkuznets@...hat.com>
Cc: stable@...r.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/kvm/hyperv.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index 23ff65504d7e..cf2a8b086fcd 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -205,7 +205,7 @@ static int synic_set_msr(struct kvm_vcpu_hv_synic *synic,
 	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 @@ 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;
@@ -514,6 +517,11 @@ static int stimer_set_config(struct kvm_vcpu_hv_stimer *stimer, u64 config,
 {
 	union hv_stimer_config new_config = {.as_uint64 = config},
 		old_config = {.as_uint64 = stimer->config.as_uint64};
+	struct kvm_vcpu *vcpu = stimer_to_vcpu(stimer);
+	struct kvm_vcpu_hv_synic *synic = vcpu_to_synic(vcpu);
+
+	if (!synic->active && (!host || config))
+		return 1;
 
 	trace_kvm_hv_stimer_set_config(stimer_to_vcpu(stimer)->vcpu_id,
 				       stimer->index, config, host);
@@ -533,6 +541,12 @@ static int stimer_set_config(struct kvm_vcpu_hv_stimer *stimer, u64 config,
 static int stimer_set_count(struct kvm_vcpu_hv_stimer *stimer, u64 count,
 			    bool host)
 {
+	struct kvm_vcpu *vcpu = stimer_to_vcpu(stimer);
+	struct kvm_vcpu_hv_synic *synic = vcpu_to_synic(vcpu);
+
+	if (!synic->active && (!host || count))
+		return 1;
+
 	trace_kvm_hv_stimer_set_count(stimer_to_vcpu(stimer)->vcpu_id,
 				      stimer->index, count, host);
 
-- 
2.31.1