I think the /etc/fstab displayed at https://docs.kernel.org/virt/uml/user_mode_linux_howto_v2.html#edit-key-system-files is missing the entry point for /dev/ubd0, the "good" line in that file should be /dev/ubd0 / ext4 discard,errors=remount-ro 0 1 In addition: - filling this bug report has no "UM" (pseudo)architecture ar the "Hardware" menu - while UML speaks about using the new vector transport, performing 'make defconfig ARCH=um' produces a .config file where CONFIG_UML_NET_VECTOR is not set.
In my case, not having / declared as mount point results in a read-only filesystem.