nomad-pledge-driver

nomad-pledge-driver is a Nomad task driver based on the pledge utility for Linux by Justine Tunney.

Security through SECCOMP sorcery

Features

• Sandbox applications by restricting syscalls they are able to make (via promises)
• Sandbox applications by allow-listing filepaths they are allowed to access (via unveil)
• Sandbox applications by restricting resources using modern Linux cgroups (via cgroups v2)
• Sandbox applications by namespace isolation using Linux namespaces (via nsenter and unshare)

Use cases

The nomad-pledge-driver is intended as a replacement for raw_exec. Sometimes there are those management tasks that just need to run as root and directly access the filesystem or perform privileged operations. While raw_exec provides no isolation, the pledge driver uses Landlock to restrict the files or directories the task is allowed to access. Specific groups of system calls are allow-listed, greatly reducing the attack surface of a mis- configured or compromised task.

Compatability

• Use version 0.3 with Nomad 1.7 and higher
• Use version 0.2 for Nomad 1.6 and below

Examples

The example below uses curl to fetch example.com, with the minimal set of promises to make a request.

More complex examples in the hack directory.

job "curl" {
  type        = "batch"

  group "group" {
    task "curl" {
      driver = "pledge"
      config {
        command  = "curl"
        args     = ["example.com"]
        promises = "stdio rpath inet dns sendfd"
        unveil   = ["r:${NOMAD_TASK_DIR}"]
      }
    }
  }
}

Building

The nomad-pledge-driver plugin is written in Go. It can be built using the normal Go toolchain steps, but the Makefile contains a dev target to make things easy. The compiled binary will appear in the output/ directory.

make dev

Installing

The plugin should be placed in the plugin_dir configured by the Nomad agent, per Nomad's documentation.

You'll also need the pledge executable (1.8 or higher) that powers the plugin sandboxing. Download the pledge executable from https://justine.lol/pledge/ and install it somewhere. The plugin configuration lets you specify where the path to the pledge executable.

sudo mkdir -p /opt/bin
curl -L -o /opt/bin/pledge-1.8.com https://justine.lol/pledge/pledge-1.8.com

👉 optional It is very convenient to bless the pledge executable with the cap_net_bind_service Linux capability. This will enable Nomad tasks using the pledge driver to bind to privileged ports (e.g. below 1024).

sudo setcap cap_net_bind_service+eip /opt/bin/pledge-1.8.com

The plugin will expose the driver.pledge.cap.net_bind attribute indicating whether the cap_net_bind_service capability has been set on the pledge-1.x.com executable.

Plugin Configuration

Currently there is only one configuration option for this plugin, which is to specify the path of the pledge executable.

plugin "nomad-pledge-driver" {
  config {
    pledge_executable = "/opt/bin/pledge-1.8.com"
  }
}

Note: in these examples the driver plugin is named pledge, and the utility executable is named pledge-1.8.com.

Task Configuration

Tasks need to specify which promises they require in order to run.

Tasks also need to unveil the filesystem paths needed to run.

For more information about which pledges are available and how this mechanism works, visit https://justine.lol/pledge/

If no user is specified for the task, the pledge plugin will use the user of the Nomad client by default. Like the raw_exec task driver, user cannot be set in hardened clusters according to the production guide.

• command: The executable to run
• args: The arguments to pass to executable
• promises: The set of promises needed for the executable to run
• unveil: The set of system filepaths to allow the task to access, and with what permission
• importance: One of lowest, low, normal, high, highest (default is normal)

# see hack/http.hcl for complete python http.server example
# note that bridge mode also works, see hack/bridge.hcl

task "task" {
  driver = "pledge"
  user   = "nobody"
  config {
    command    = "python3"
    args       = ["-m", "http.server", "${NOMAD_PORT_http}", "--directory", "${NOMAD_TASK_DIR}"]
    promises   = "stdio rpath inet"
    unveil     = ["r:/etc/mime.types", "r:${NOMAD_TASK_DIR}"]
    importance = "low"
  }

  template {
    destination = "local/index.html"
    data        = <<EOH
<!doctype html>
<html>
  <title>example</title>
  <body><p>Hello, friend!</p></body>
</html>
EOH
  }
}

Troubleshooting

For help getting the plugin to work, see the TROUBLESHOOT doc. Otherwise feel free to file an issue!

Contributing

The nomad-pledge-driver plugin is currently under active development - anything may change at a moments notice!

hacking

The included Makefile includes helpful targets for hacking on the pledge plugin.

To simply compile, run make dev. The output will go into /tmp/plugins.

To start Nomad with the plugin, run make run. Under the hood this is using the hack/client.hcl Client config file, along with -dev mode defaults. You should be able to run jobs making use of pledge driver when launching Nomad this way.

There are example jobs in the hack/ directory.

License

The pledge task driver plugin is made open source under the MPL-2.0 license.