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Nabla containers: a new approach to container isolation

Despite all of the advantages that have resulted in an industry-wide shift towards containers, containers have not been accepted as isolated sandboxes, which is crucial for container-native clouds. We introduce nabla containers, a new type of container designed for strong isolation on a host.

Nabla containers achieve isolation by adopting a strategy of attack surface reduction to the host. A visualization of this approach appears in this figure:

nabla-containers

A containerized application can avoid making a Linux system call if it links to a library OS component that implements the system call functionality. Nabla containers use library OS (aka unikernel) techniques, specifically those from the Solo5 project, to avoid system calls and thereby reduce the attack surface. Nabla containers only use 7 system calls; all others are blocked via a Linux seccomp policy. An overview of the internals of a nabla container appears in this figure:

nabla-internals

For the curious, here are the allowed syscalls: read, write, exit_group, clock_gettime, ppoll, pwrite64, and pread64. They are restricted to specific file descriptors (already opened before enabling seccomp). They originate from the hypercall implementations of the ukvm unikernel monitor.1 Check out the code for more specifics.

Are nabla containers really more isolated?

The isolation in nabla containers comes from limiting access to the host kernel via the blocking of system calls. We have measured exactly how much access to the kernel common applications exhibit with nabla containers and standard containers by measuring the number of system calls containerized applications make and correspondingly how many kernel functions they access. This graph summarizes results for a few applications:

nabla-isolation

Further measurements and results and scripts to reproduce them reside in the nabla-measurements repository.

Repository overview

More information appears in each of the individual repositories related to nabla containers. In addition, this article steps you through the process of running your first nabla container:

If you want to go deeper, check out the following repositories:

Limitations

The main limitation is that the Nabla runtime (runnc) only supports images built for nabla (see nabla-base-build). Other limitations are listed here.

  1. For more information about ukvm, check out our HotCloud ‘16 paper Unikernel Monitors: Extending Minimalism Outside of the Box or the Solo5 project on Github.