Customers often ask how Prisma Cloud Defender really works under the covers. Prisma Cloud leverages Docker’s ability to grant advanced kernel capabilities to enable Defender to protect your whole stack, while being completely containerized and utilizing a least privilege security design.
Because we’ve built Prisma Cloud expressly for cloud native stacks, the architecture of our agent (what we call Defender) is quite different. Rather than having to install a kernel module, or modify the host OS at all, Defender instead runs as a Docker container and takes only those specific system privileges required for it to perform it’s job. It does not run as --privileged and instead takes the specific system capabilities of net_admin, sys_admin, sys_ptrace, and audit_control that it needs to be able to enter the host namespace and interact with both it and other containers running on the system. You can see this clearly by inspecting the Defender container:
# docker inspect twistlock_defender_<VERSION> | grep -e CapAdd -A 5 -e Priv "CapAdd": [ "NET_ADMIN", "SYS_ADMIN", "SYS_PTRACE", "AUDIT_CONTROL" ], -- "Privileged": false,
This architecture allows Defender to have a near real time view of the activity occurring at the kernel level. Because we also have detailed knowledge of the operations of each container, we can correlate the kernel data with the container data to get a comprehensive view of process, file system, network, and system call activity from the kernel and all the containers running on it. This access also allows us to take preventative actions like stopping compromised containers and blocking anomalous processes and file system writes.
Critically, though, Defender runs as a user mode process. If Defender were to fail (and if that were to happen, it would be restarted immediately), there would be no impact on the containers on the host, nor the host kernel itself. Additionally, we can and do apply cgroups to set hard limits on CPU and memory consumption, guaranteeing it will be a ‘good neighbor’ on the host and not interfere with host performance or stability.
In the event of a communications failure with Console, Defender continues running and enforcing the active policy that was last pushed by the management point. Events that would be pushed back to Console are cached locally until it is once again reachable.
Why not a kernel module?
Given the broad range of security protection Prisma Cloud provides, not just for containers, but also for the hosts they run on, you might assume that we use a kernel module - with all the associated baggage that goes along with that. However, that’s not actually how Prisma Cloud works.
Kernel modules are compiled software components that can be inserted into the kernel at runtime and typically provide enhanced capabilities for low level functionality like process scheduling or file monitoring. Because they run as part of the kernel, these components are very powerful and privileged. This allows them to perform a wide range of functions but also greatly increases the operational and security risks on a given system. The kernel itself is extensively tested across broad use cases, while these modules are often created by individual companies with far fewer resources and far more narrow test coverage.
Because kernel modules have unrestricted system access, a security flaw in them is a system wide exposure. A single unchecked buffer or other error in such a low level component can lead to the complete compromise of an otherwise well designed and hardened system. Further, kernel modules can introduce significant stability risks to a system. Again, because of their wide access, a poorly performing kernel module that’s frequently called can drag down performance of the entire host, consume excessive resources, and lead to kernel panics. For these reasons, many modern operating systems designed for cloud native apps, like Google Container-Optimized OS, explicitly prevent the usage of kernel modules.
Defender is responsible for enforcing vulnerability and compliance blocking rules. When a blocking rule is created, Defender moves the original runC binary to a new path and inserts a Prisma Cloud runC shim binary in its place.
When a command to create a container is issued, it propagates down the layers of the container orchestration stack, eventually terminating at runC. Regardless of your environment (Docker, Kubernetes, or OpenShift, etc) and underlying CRI provider, runC does the actual work of instantiating a container.
When starting a container in a Prisma Cloud-protected environment:
- The Prisma Cloud runC shim binary intercepts calls to the runC binary.
- The shim binary calls the Defender container to determine whether the new container should be created based on the installed policy.
- If Defender replies affirmatively, the shim calls the original runC binary to create the container, and then exits.
- If Defender replies negatively, the shim terminates the request.
- If Defender does not reply within 60 seconds, the shim calls the original runC binary to create the container and then exits.
The last step guarantees that Defender always fails open, which is important for the resiliency of your environment. Even if the Defender process terminates, becomes unresponsive, or cannot be restarted, a failed Defender will not hinder deployments or the normal operation of a node.
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