In this lesson, you will learn how to enforce strict identity boundaries to ensure that users in one application environment remain completely isolated from those in another on the same shared host.
The Challenge of Shared Environments
In a multi-tenant Linux host, simply creating different user accounts is rarely enough. If two applications (e.g., app-alpha and app-beta) run on the same kernel, a compromised user process in app-alpha could potentially view, signal, or trace processes belonging to app-beta. To maintain a strict identity boundary, we must move beyond standard Unix permissions and implement hardware-level and kernel-level isolation.
Implementing Identity Boundaries
To enforce these boundaries, we combine user namespace mapping with process hiding. By restricting process visibility, we prevent a user from gathering reconnaissance on other tenants.
First, ensure that your kernel is configured to hide processes from other users. You can modify /etc/fstab to mount the /proc filesystem with specific hide options:
# Add this to /etc/fstab
proc /proc proc defaults,hidepid=2 0 0
With hidepid=2, users can only see their own processes. Even if a user from app-alpha manages to run a command, they will see an empty process list for any user associated with app-beta.
Next, you should pair this with Cgroups (Control Groups) to limit the resources available to each tenant. By assigning each application's service user to a specific cgroup, you create an effective "sandbox" that limits CPU and memory usage, preventing a "noisy neighbor" from crashing the entire host.
Finally, ensure your PAM configuration uses pam_namespace.so. This allows you to create private /tmp and /var/tmp directories for each user session, ensuring that even if two users share the same underlying hardware, their temporary file interactions are cryptographically and logically isolated from one another.
By combining process hiding, cgroup-based resource quotas, and private session namespaces, you effectively turn a single physical host into a series of hardened, independent "virtual" environments.
Key Takeaway: Strict multi-tenancy requires shifting from a shared-system mindset to a siloed-process model using hidepid, Cgroups, and private PAM namespaces.