The XSELINUX code in the Xserver labels the X resources based on a hook. What happens here is that the GLX PBuffer code does not call that XACE hook when creating the buffer, so it remains unlabeled, and when the client issues another request to access that resource (as here with a GetGeometry) or even when it creates another resource which needs to access that buffer (such as a GC), the XSELINUX code will try to use an object that was never labeled and crash because the SID is NULL.