Here is my situation. I have a host computer running ubuntu server 14.04 It mounts a dev from fstab:
/dev/sdb2 /media ext4 rw,nosuid,nodev 0 0
Permissions on the mounted folder are owner=root, group=root, view content, change content, and accesss content are all =anyone.
Everything on the host machine works fine.
The host has a virtual ubuntu server 14.04 created with virt-manager. In virtmanager I create a writable filesystem mode=passthrough, driver=default, source=/media, target=hostmedia
On this VM I mount the fiesystem in fstab with:
hostmedia /media 9p trans-virtio,_netdev, rw, acl 0 0
On this guest VM, only root can create/modify/delete files. Every other user has read only. I can't figure out how to change this.