[PATCH RFC 22/22] KVM: x86: hyper-v: Check access to HVCALL_NOTIFY_LONG_SPIN_WAIT hypercall

[Vitaly Kuznetsov]

TLFS6.0b states that partition issuing HVCALL_NOTIFY_LONG_SPIN_WAIT must
posess 'UseHypercallForLongSpinWait' privilege but there's no
corresponding feature bit. Instead, we have "Recommended number of attempts
to retry a spinlock failure before notifying the hypervisor about the
failures. 0xFFFFFFFF indicates never notify." Use this to check access to
the hypercall. Also, check against zero as the corresponding CPUID must
be set (and '0' attempts before re-try is weird anyway).

Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
---
 arch/x86/kvm/hyperv.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index 37b8ff30fc1d..325446833bbe 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -2113,6 +2113,12 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
 
 	switch (code) {
 	case HVCALL_NOTIFY_LONG_SPIN_WAIT:
+		if (unlikely(!hv_vcpu->cpuid_cache.enlightenments_ebx ||
+			     hv_vcpu->cpuid_cache.enlightenments_ebx == U32_MAX)) {
+			ret = HV_STATUS_ACCESS_DENIED;
+			break;
+		}
+
 		if (unlikely(rep)) {
 			ret = HV_STATUS_INVALID_HYPERCALL_INPUT;
 			break;
-- 
2.30.2