[PATCH v2 7/8] KVM: x86: Reject fixeds-size Hyper-V hypercalls with non-zero "var_cnt"

[Sean Christopherson <seanjc@xxxxxxxxxx>]

Reject Hyper-V hypercalls if the guest specifies a non-zero variable size
header (var_cnt in KVM) for a hypercall that has a fixed header size.
Per the TLFS:

  It is illegal to specify a non-zero variable header size for a
  hypercall that is not explicitly documented as accepting variable sized
  input headers. In such a case the hypercall will result in a return
  code of HV_STATUS_INVALID_HYPERCALL_INPUT.

Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/kvm/hyperv.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index 3d83d6a5d337..ad455df850c9 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -2241,14 +2241,14 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
 
 	switch (hc.code) {
 	case HVCALL_NOTIFY_LONG_SPIN_WAIT:
-		if (unlikely(hc.rep)) {
+		if (unlikely(hc.rep || hc.var_cnt)) {
 			ret = HV_STATUS_INVALID_HYPERCALL_INPUT;
 			break;
 		}
 		kvm_vcpu_on_spin(vcpu, true);
 		break;
 	case HVCALL_SIGNAL_EVENT:
-		if (unlikely(hc.rep)) {
+		if (unlikely(hc.rep || hc.var_cnt)) {
 			ret = HV_STATUS_INVALID_HYPERCALL_INPUT;
 			break;
 		}
@@ -2258,7 +2258,7 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
 		fallthrough;	/* maybe userspace knows this conn_id */
 	case HVCALL_POST_MESSAGE:
 		/* don't bother userspace if it has no way to handle it */
-		if (unlikely(hc.rep || !to_hv_synic(vcpu)->active)) {
+		if (unlikely(hc.rep || hc.var_cnt || !to_hv_synic(vcpu)->active)) {
 			ret = HV_STATUS_INVALID_HYPERCALL_INPUT;
 			break;
 		}
@@ -2271,14 +2271,14 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
 				kvm_hv_hypercall_complete_userspace;
 		return 0;
 	case HVCALL_FLUSH_VIRTUAL_ADDRESS_LIST:
-		if (unlikely(!hc.rep_cnt || hc.rep_idx)) {
+		if (unlikely(!hc.rep_cnt || hc.rep_idx || hc.var_cnt)) {
 			ret = HV_STATUS_INVALID_HYPERCALL_INPUT;
 			break;
 		}
 		ret = kvm_hv_flush_tlb(vcpu, &hc, false);
 		break;
 	case HVCALL_FLUSH_VIRTUAL_ADDRESS_SPACE:
-		if (unlikely(hc.rep)) {
+		if (unlikely(hc.rep || hc.var_cnt)) {
 			ret = HV_STATUS_INVALID_HYPERCALL_INPUT;
 			break;
 		}
@@ -2299,7 +2299,7 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
 		ret = kvm_hv_flush_tlb(vcpu, &hc, true);
 		break;
 	case HVCALL_SEND_IPI:
-		if (unlikely(hc.rep)) {
+		if (unlikely(hc.rep || hc.var_cnt)) {
 			ret = HV_STATUS_INVALID_HYPERCALL_INPUT;
 			break;
 		}
@@ -2331,6 +2331,11 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
 			ret = HV_STATUS_OPERATION_DENIED;
 			break;
 		}
+		if (unlikely(hc.var_cnt)) {
+			ret = HV_STATUS_INVALID_HYPERCALL_INPUT;
+			break;
+		}
+
 		vcpu->run->exit_reason = KVM_EXIT_HYPERV;
 		vcpu->run->hyperv.type = KVM_EXIT_HYPERV_HCALL;
 		vcpu->run->hyperv.u.hcall.input = hc.param;
-- 
2.33.1.1089.g2158813163f-goog