Is there a way to restrict access to directories within a Kubernetes container?

If I have two groups that are not root users that will access a container's directory structure, is there a way to fine tune permissions such that Group 1 can have WRITE permissions on /DIR1, but Group 2 only has READ or even NO ACCESS permissions on /DIR1? Assuming that this /DIR1 is NOT A MOUNTED VOLUME?

Does the answer change if the directory IS a mounted volume?

I am unable to find an absolute answer online, but I think I might be touching on something called a security context, though I can't quite wrap my head around it, so I don't know if I am understanding it correctly as the examples always show a root, and a non-root user. But never two non-root users.

I have considered the following avenues:

RoleBindings, but I am unable to find how I can limit or tweak something like the existing Read-Only role to point to specific directories? It seems to read K8 resources.
I cannot completely remove all roles from Group 2 as they will have to access the pods at some point to troubleshoot. Maybe.
I know you can chmod / chown in the dockerfile during image build, but.... not sure how this would tie into users that log in and a variety of groups that may need to access the same directory. Like what if Group 1 and Group 3 need access? Can you chown 2 groups? Does it even work like that?

Solution

In your Dockerfile, create groups/users and set strict permissions:

RUN groupadd group1 && groupadd group2 && \
useradd -g group1 user1 && useradd -g group2 user2 && \
mkdir /DIR1 && \
chown user1:group1 /DIR1 && \  # Owned by user1 and group1
chmod 770 /DIR1  # rwx for owner/group, no access for others

In the pod’s YAML, set the runtime identity:

securityContext:
runAsUser: 1000
runAsGroup: 1000

Use fsGroup to set volume group:

securityContext:
fsGroup: 1000

(if you want to) Use an initContainer to fix permissions:

initContainers:
 - name: fix-permissions
   image: busybox
   command: ["sh", "-c", "chmod 770 /DIR1"]
   volumeMounts:
    - name: my-volume
      mountPath: /DIR1