Nomad exec2 Driver

The exec2 task driver plugin is a modern alternative to Nomad's original exec and raw_exec drivers. It offers a security model optimized for running 'ordinary' processes with very low startup times and minimal overhead in terms of CPU and memory utilization. exec2 leverages kernel features such as the Landlock LSM, cgroups v2, and ordinary file system permissions.

Requirements

• Linux 5.15+
• Cgroups v2 enabled
• Landlock LSM enabled
• Commands unshare and nsenter
• Nomad client running as root

Recent mainstream Linux distributions such as Ubuntu 22.04 and RHEL 9 meet the requirements and are well supported.

Example Jobs

Here is a simple example running env. It makes use of a dynamic workload user and does not require any extra filepaths to unveil.

job "env" {
  type = "batch"
  group "group" {
    task "task" {
      driver = "exec2"
      config {
        command = "env"
      }
    }
  }
}

Concepts

Filesystem Isolation

landlock

The exec2 driver makes use of go-landlock for providing filesystem isolation, making the host filesystem unreachable except where explicitly allowed.

By default a task is enabled to access its task directory and its shared alloc directory. The paths to these directories are accessible by reading the environment variables $NOMAD_TASK_DIR and $NOMAD_ALLOC_DIR respectively.

A file access mode must also be specified when granting additional filesystem access to a path. This is done by prefixing the path with 'r', 'w', 'x', and/or 'c' indicating read, write, executable, and create permissions, respectively. e.g.,

• r:/srv/www - read-only access to /srv/www
• rwc:/tmp - read, write, and create files in /tmp
• rx:/opt/bin/application - read and execute a specific application
• wc:/var/log - write and create files in /var/log

This style of permission control is modeled after the unveil system call introduced by the OpenBSD project. In configuration parameters we refer to the "unveil"-ing of filesystem paths as exec2 is leveraging landlock to emulate the semantics of unveil.

dynamic workload users

While landlock prevents tasks from accessing the host filesystem, Nomad 1.8 introduces dynamic workload users which enable tasks to be run as a PID/GID that is not assigned to any user. This provides protection from non-root users getting access inside the task and allocation directories created for the task.

To make use of a dynamic workload user, simply leave the user field blank in the task definition of an exec2 task.

Resource Isolation

Similar to exec and other container runtimes, exec2 makes use of cgroups for limiting the amount of CPU and RAM a task may consume.

Configuration

Plugin Configuration

The default plugin configuration is shown below. System default paths are enabled, but nothing else. These default paths enable basic functionality like reading system TLS certificates, executing programs in /bin, /usr/bin, and accessing shared object files. The exact set of default paths is system dependent, and can be disabled or customized in plugin config.

The default set of default paths are listed below. These paths are enabled only if they are found to exist at the time of the task launching.

shared objects

• /dev/null (read, write)
• /lib (read, execute)
• /lib64 (read, execute)
• /usr/lib (read, execute)
• /usr/libexec (read, execute)
• /usr/local/lib (read, execute)
• /usr/local/lib64 (read, execute)
• /etc/ld.so.conf (read)
• /etc/ld.so.cache (read)
• /etc/ld.so.conf.d (read)

io, common

• /tmp (read, write, create)
• /dev/full (read, write)
• /dev/zero (read)
• /dev/fd (read)
• /dev/stdin (read, write)
• /dev/stdout (read, write)
• /dev/urandom (read)
• /dev/log (write)
• /usr/share/locale (read)
• /proc/self/cmdline (read)
• /usr/share/zoneinfo (read)
• /usr/share/common-licenses (read)
• /proc/sys/kernel/ngroups_max (read)
• /proc/sys/kernel/cap_last_cap (read)
• /proc/sys/vm/overcommit_memory (read)

dns

• /etc/hosts (read)
• /hostname (read)
• /etc/services (read)
• /etc/protocols (read)
• /etc/resolv.conf (read)

certificates

• /etc/ssl/certs (read)
• /etc/pki/tls/certs (read)
• /sys/etc/security/cacerts (read)
• /etc/ssl/ca-bundle.pem (read)
• /etc/pki/tls/cacert.pem (read)
• /etc/pki/ca-trust-extracted/pem/tls-ca-bundle.pem (read)
• /etc/ssl/cert.pem (read)

Additional allowable paths can be specified at the plugin level, which applies to all tasks making use of the exec2 driver, or at the task level, which will apply specifically to each task.

plugin "nomad-driver-exec2" {
  config {
    unveil_defaults = true
    unveil_paths    = []
    unveil_by_task  = false
  }
}

• unveil_defaults - (default: true) - enable or disable default system paths useful for running basic commands

• unveil_paths - (default: []) - a list of filesystem paths with permissions to grant to all tasks

unveil_paths = ["rx:/opt/bin", "r:/srv/certs"]

• unveil_by_task - (default: false) - enable or disable job submitters to specify additional filesystem path access within task config

Task Configuration

config

Task configuration for an exec2 task includes setting a command, args for the command, and additional unveil paths if unveil_by_task is enabled in plugin configuration.

config {
  command = "/usr/bin/cat"
  args    = ["/etc/os-release"]
  unveil  = ["r:/etc/os-release"]
}

• command - (required) - command to run

• args - (optional) - list of arguments to provide to command

• unveil - (optional) - list of additional filesystem paths to provide access to the task (requires unveil_by_task in plugin config)

cpu

Tasks can be limited in CPU resources by setting the cpu or cores values in the task resources block.

• cpu - (default: 100) - limits the CPU bandwidth allowable for the task to make use of in MHz, may not be used with cores

• cores - (optional) - specifies the number of CPU cores to reserve exclusively for the task, may not be used with cpu

memory

Tasks can be limited in memory resources by setting memory and optionally the memory_max values in the task resources block.

• memory - (default: 300) - specifies the memory required in MB

• memory_max - (optional) - specifies the maximum memory the task may use if the client has excess memory capacity and memory oversubscription is enabled for the cluster/node pool.

Attributes

When installed, the exec2 plugin provides the following node attributes which can be used as constraints when authoring jobs.

driver.exec2.unveil.defaults    = true
driver.exec2.unveil.tasks       = true

Install

The exec2 driver is an external Nomad task-driver plugin. It can be compiled from source or downloaded from HashiCorp.

See the Nomad Task Driver documentation for getting started with using external task driver plugins.

Hacking

For local development, the included [Makefile] includes a hack target which builds the plugin and launches Nomad in -dev mode with the plugin directory set to include the development build of the plugin.

There are two test suites - one for unit tests and one for a small e2e test suite which runs the plugin through a real Nomad client. To run the e2e suite, set GOFLAGS=-tags=e2e and run go test in the e2e directory. A Nomad client must already be running.

make hack