Evolving Container Security With Linux User Namespaces
By: Netflix Technology Blog
Tags: Security, Docker, Containers, Kubernetes

By Fabio Kung, Sargun Dhillon, Andrew Spyker, Kyle Anderson, Rob Gulewich, Nabil Schear, Andrew Leung, Daniel Muino, and Manas Alekar

As previously discussed on the Netflix Tech Blog, Titus is the Netflix container orchestration system. It runs a wide variety of workloads from various parts of the company — everything from the frontend API for netflix.com, to machine learning training workloads, to video encoders. In Titus, the hosts that workloads run on are abstracted from our users. The Titus platform maintains large pools of homogenous node capacity to run user workloads, and the Titus scheduler places workloads. This abstraction allows the compute team to influence the reliability, efficiency, and operability of the fleet via the scheduler. The hosts that run workloads are called Titus “agents.” In this post, we describe how Titus agents leverage user namespaces to improve the overall security of the Titus agent fleet.

Titus’s Multi-Tenant Clusters

The Titus agent fleet appears to users as a homogenous pool of capacity. Titus internally employs a cellular bulkhead architecture for scalability, so the fleet is composed of multiple cells. Many bulkhead architectures partition their cells on tenants, where a tenant is defined as a team and their collection of applications. We do not take this approach, and instead, we partition our cells to balance load. We do this for reliability, scalability, and efficiency reasons.

Titus is a multi-tenant system, allowing multiple teams and users to run workloads on the system, and ensuring they can all co-exist while still providing guarantees about security and performance. Much of this comes down to isolation, which comes in multiple forms. These forms include performance isolation (ensuring workloads do not degrade one another’s performance), capacity isolation (ensuring that a given tenant can acquire resources when they ask for them), fault isolation (ensuring that the failure of a part of the system doesn’t cause the whole system to fail), and security isolation (ensuring that the compromise of one tenant’s workload does not affect the security of other tenants). This post focuses on our approaches to security isolation.

Secure Multi-tenancy

One of Titus’s biggest concerns with multi-tenancy is security isolation. We want to allow different kinds of containers from different tenants to run on the same instance. Security isolation in containers has been a contentious topic. Despite the risks, we’ve chosen to leverage containers as part of our security boundary. To offset the risks brought about by the container security boundary, we employ some additional protections.

The building blocks of multi-tenancy are Linux namespaces, the very technology that makes LXC, Docker, and other kinds of containers possible. For example, the PID namespace makes it so that a process can only see PIDs in its own namespace, and therefore cannot send kill signals to random processes on the host. In addition to the default Docker namespaces (mount, network, UTS, IPC, and PID), we employ user namespaces for added layers of isolation. Unfortunately, these default namespace boundaries are not sufficient to prevent container escape, as seen in CVEs like CVE-2015–2925. These vulnerabilities arise due to the complexity of interactions between namespaces, a large number of historical decisions during kernel development, and leaky abstractions like the proc filesystem in Linux. Composing these security isolation primitives correctly is difficult, so we’ve looked to other layers for additional protection.

Running many different workloads multi-tenant on a host necessitates the prevention lateral movement, a technique in which the attacker compromises a single piece of software running in a container on the system, and uses that to compromise other containers on the same system. To mitigate this, we run containers as unprivileged users — making it so that users cannot use “root.” This is important because, in Linux, UID 0 (or root’s privileges), do not come from the mere fact that the user is root, but from capabilities. These capabilities are tied to the current process’s credentials. Capabilities can be added via privilege escalation (e.g., sudo, file capabilities) or removed (e.g., setuid, or switching namespaces). Various capabilities control what the root user can do. For example, the CAP_SYS_BOOT capability controls the ability of a given user to reboot the machine. There are also more common capabilities that are granted to users like CAP_NET_RAW, which allows a process the ability to open raw sockets. A user can automatically have capabilities added when they execute specific files via file capabilities. For example, on a stock Ubuntu system, the ping command needs CAP_NET_RAW:

One of the most powerful capabilities in Linux is CAP_SYS_ADMIN, which is effectively equivalent to having superuser access. It gives the user the ability to do everything from mounting arbitrary filesystems, to accessing tracepoints that can expose vital information about the Linux kernel. Other powerful capabilities include CAP_CHOWN and CAP_DAC_OVERRIDE, which grant the capability to manipulate file permissions.

In the kernel, you’ll often see capability checks spread throughout the code, which looks something like this:

Notice this function doesn’t check if the user is root, but if the task has the CAP_SYS_ADMIN capability before allowing it to execute.

Docker takes the approach of using an allow-list to define which capabilities a container receives. These can be extended or attenuated by the user. Even the default capabilities that are defined in the Docker profile can be abused in certain situations. When we looked into running workloads as unprivileged users without many of these capabilities, we found that it was a non-starter. Various pieces of software used elevated capabilities for FUSE, low-level packet monitoring, and performance tracing amongst other use cases. Programs will usually start with capabilities, perform any activities that require those capabilities, and then “drop” them when the process no longer needs them.

User Namespaces

Fortunately, Linux has a solution — User Namespaces. Let’s go back to that kernel code example earlier. The pcrlock function called the capable function to determine whether or not the task was capable. This function is defined as:

This checks if the task has this capability relative to the init_user_ns. The init_user_ns is the namespace that processes are initialially spawned in, as it’s the only user namespace that exists at kernel startup time. User namespaces are a mechanism to split up the init_user_ns UID space. The interface to set up the mappings is via a “uid_map” and “gid_map” that’s exposed via /proc. The mapping looks something like this:

This allows UIDs in user-namespaced containers to be mapped to host UIDs. A variety of translations occur, but from the container’s perspective, everything is from the perspective of the UID ranges (otherwise known as extents) that are mapped. This is powerful in a few ways:

It allows you to make certain UIDs off-limits to the container — if a UID is not mapped in the user namespace to a real UID, and you try to examine a file on disk with it, it will show up as overflowuid / overflowgid, a UID and GID specified in /proc/sys to indicate that it cannot be mapped into the current working space. Also, the container cannot setuid to a UID that can access files owned by that “outside uid.” From the user namespace’s perspective, the container’s root user appears to be UID 0, and the container can use the entire range of UIDs that are mapped into that namespace. Kernel subsystems can then proceed to call ns_capable with the specific user namespace that is tied to the resource. Many capability checks are now done to a user namespace that is relative to the resource being manipulated. This, in turn, allows processes to exercise certain privileges without having any privileges in the init user namespace. Even if the mapping is the same across many different namespaces, capability checks are still done relative to a specific user namespace.

One critical aspect of understanding how permissions work is that every namespace belongs to a specific user namespace. For example, let’s look at the UTS namespace, which is responsible for controlling the hostname:

The namespace has a relationship with a particular user namespace. The ability for a user to manipulate the hostname is based on whether or not the process has the appropriate capability in that user namespace.

Let’s Get Into It

We can examine how the interaction of namespaces and users work ourselves. To set the hostname in the UTS namespace, you need to have CAP_SYS_ADMIN in its user namespace. We can see this in action here, where an unprivileged process doesn’t have permission to set the hostname: