Writing to Bind Mounts from Unprivileged LXC Containers

Update: As evident by the comments on this post, it seems this method may not work for all installs. The issue seems to be with SMB shares. This post outlines the process with NFS shares. Thanks to Alex in the comments for these findings. If you find other issues or learn why this is the case, leave a comment below or fill out the contact form.

Containers vs VMs

When ever I run into an issue during GNU/Linux server administration, 9 times out of 10, it’s due to permissions. By this point, it’s only frustrating when I realize that I didn’t check the permissions first. Since starting my homelab years ago, one issue that has plagued me has been giving write access to my unprivileged LXC containers in a shared storage.

I could possibly sidestep this problem by starting a VM but I like containers. Why? Containers are great because they reduce resources consumed, segment logic, and are quickly reproduced. This is all accomplished by using existing features of the Linux kernel and its user space. The host machine already has a kernel (unlike a VM which is given its own kernel), so when running a container, the host machine kernel is shared with the container and is managed by the host as another user on the system. By design, unprivileged LXC containers (henceforth known as unpriv LXC) have no permissions on the host machine. They are relegated to the nobody user and nogroup group. This ensures that if an attacker were to compromise the container, they would have no permissions on the host machine. That’s all well and good, but what if you want to share storage across your unprivileged containers? There are presumably ways that you can punch holes in AppArmor but Proxmox does in fact, make it simple.

In my example, I have an unpriv LXC running Plex. I don’t want this container to be able to do anything on the host but I would like it to be able to read and write from a shared media folder on my network.

Mounting NFS

The first step, is making sure the Proxmox host has access to the Network File Share (NFS). This is incredibly simple via the GUI. Under Storage View, click the Datacenter, then click Storage. Click the “Add” button and select the appropriate storage option. In your case, it may be SMB/CIFS or Directory but in my case, it’s NFS.

Bind Mount

Once our host has access to the NFS, we need to give the container access to that data via a bind mount. A bind mount is a folder on the host that is mapped inside the container. To create the bind mount, open the Proxmox CLI, and run

pct set 100 -mp0 /host/shared_dir_location,mp=/path/in/container

• pct is the Proxmox Container Toolkit

• set tells pct we’re going to set an option

• 100 is the container ID we’ll be working on

• -mp0 is the name of the mount point

• the first path listed is the directory on your host you’re attempting to share with the container

• ,mp=/path is the path where we want that directory mounted within the container

Permissions

From here, open the Proxmox GUI (web interface) and within the Server View, click Datacenter > Permissions > Groups.

After that, click the “Create” button to make a new group permission. Give it a name like “shared_file_access” and a description so you know what it does. We’ll also go on to select “Roles” below “Groups” and create a new role. Give the new role a name like “DataAccessRole” and assign it the storage related privileges, which are the ones that start with “Datastore.” as well as “Pool.Allocate” and “Pool.Audit.”

Once that’s complete, we can select our container from the server list on the left, navigate to it’s permissions, and click “Add” to give it our group permission and the role. We’ll want to do the same thing for the storage point we mounted earlier on the host. That can be found under Data center > Storage. Select your storage point, and navigate to the permissions of it. From there, give it the same permissions you gave to our container.

If your storage location is properly mounted inside of Proxmox, your unpriv LXC should now be able to read and write to the location we mounted earlier! You can test this by opening a terminal in your unpriv LXC, navigating to the bind mount point, and attempting to create a file there eg. touch test.txt.

Comment below with how well this worked for you, and if you liked this post, share it around.