better custom validation for angular form

I searched, I know 'the directive way' to do custom validation, but it looks like a overkill to me, I hope there is a simpler way, so I tried somwthing like this.

it works, but really not sure it is the right way(or angular way).

.controller('vali', ['$scope', function vali($scope) {

    var th = this

    th.formData = {}

    // watch password repeat
    $scope.$watch(function() {
        return th.password1
    }, function(newPass) {
        if(!customValidate(newPass)) th.form.pass2.$setValidity('custom', false)
        else th.form.pass2.$setValidity('custom', true)
    })

    //custom validate: password repeat must equal to password
    function customValidate(v) {
        if(v === th.formData.password) return true
        else return false
    }

    this.submit = function(form) {
        if(!th.form.$invalid) th.postToServer(th.formData)
    }

    this.postToServer = function(data) {
        //do post to server
        console.log(data)
    }

}])

html:

<div ng-controller='vali as v'>
  <form name='v.form' ng-submit='v.submit'>
    <input type='password' name='pass1' ng-model='v.formData.password' minlength='6' />
    <input type='password' name='pass2' ng-model='v.password1' />

    <div ng-messages='v.form.pass2.$error' ng-if='v.form.pass2.$dirty'>
        <div ng-message='required'>required</div>
        <div ng-message='custom'> not equal</div>
    </div>

    <button type='submit'>submit</button>
  </form>
</div>