[PATCH v3 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.

Note, at least some of the various DEBUG commands likely aren't allowed
to use variable size headers, but the TLFS documentation doesn't clearly
state what is/isn't allowed.  Omit them for now to avoid unnecessary
breakage.

Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/kvm/hyperv.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index f33a5e890048..522ccd2f0db4 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -2250,14 +2250,14 @@ 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;
 		}
@@ -2267,7 +2267,7 @@ 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;
 		}
@@ -2280,14 +2280,14 @@ 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;
 		}
@@ -2308,7 +2308,7 @@ 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;
 		}
-- 
2.34.1.400.ga245620fadb-goog