[seL4] testing rust program in sel4webserver example project

Hi, I am trying to compile a simple rust web server to be executed in the virtual machine (as lighthttpd works in this webserver project). This server works perfect (I am using qemu for testing -aarch64), but when I try to get some random bytes using the rand crate the program hangs:

.... let mut key = vec![0u8; 32]; rand::thread_rng().fill_bytes(&mut key); ...

Do anybody knows if this could be related to some missing settings (in qemu command line or in some device.camkes file) or some rust features that don't work in a sel4 VM??

Thanks