KVM: hyperv: idr_find needs RCU protection

Even though the eventfd is released after the KVM SRCU grace period
elapses, the conn_to_evt data structure itself is not; it uses RCU
internally, instead.  Fix the read-side critical section to happen
under rcu_read_lock/unlock; the result is still protected by
vcpu->kvm->srcu.
Reviewed-by: NRoman Kagan <rkagan@virtuozzo.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/hyperv.c

@@ -1296,8 +1296,10 @@ static u16 kvm_hvcall_signal_event(struct kvm_vcpu *vcpu, bool fast, u64 param)
 	if (param & ~KVM_HYPERV_CONN_ID_MASK)
 		return HV_STATUS_INVALID_HYPERCALL_INPUT;
 
-	/* conn_to_evt is protected by vcpu->kvm->srcu */
+	/* the eventfd is protected by vcpu->kvm->srcu, but conn_to_evt isn't */
+	rcu_read_lock();
 	eventfd = idr_find(&vcpu->kvm->arch.hyperv.conn_to_evt, param);
+	rcu_read_unlock();
 	if (!eventfd)
 		return HV_STATUS_INVALID_PORT_ID;