How should I get the far plane vertex data of the view frustum

Question

I want to get the vertex data of the far plane of the view frustum, but I found that in cesium, I can’t get the correct upward direction of the view frustum I created. I want to know how to get the four far planes of any view frustum. The coordinates of the vertex

text

If there are multiple frustums in cesium, the upward direction of the frustum camera cannot be correctly obtained.