nomad/client/allocrunner/taskrunner/getter/util_linux.go

// Copyright (c) HashiCorp, Inc.
// SPDX-License-Identifier: BUSL-1.1

//go:build linux

package getter

import (
	"os"
	"path/filepath"

	"github.com/mitchellh/go-homedir"
	"github.com/shoenig/go-landlock"
	"golang.org/x/sys/unix"
)

// findHomeDir returns the home directory as provided by os.UserHomeDir. In case
// os.UserHomeDir returns an error, we return /root if the current process is being
// run by root, or /dev/null otherwise.
func findHomeDir() string {
	// When running as a systemd unit the process may not have the $HOME
	// environment variable set, and os.UserHomeDir will return an error.

	// if home is set, just use that
	if home, err := homedir.Dir(); err == nil {
		return home
	}

	// if we are the root user, use "/root"
	if unix.Getuid() == 0 {
		return "/root"
	}

	// nothing safe to do
	return "/nonexistent"
}

// defaultEnvironment is the default minimal environment variables for Linux.
func defaultEnvironment(taskDir string) map[string]string {
	tmpDir := filepath.Join(taskDir, "tmp")
	homeDir := findHomeDir()
	return map[string]string{
		"PATH":   "/usr/local/bin:/usr/bin:/bin",
		"TMPDIR": tmpDir,
		"HOME":   homeDir,
	}
}

// lockdownAvailable returns if lockdown is implemented for
// the current platform.
func lockdownAvailable() bool {
	return landlock.Available()
}

// lockdown isolates this process to only be able to write and
// create files in the task's task directory.
// dir - the task directory
//
// Only applies to Linux, when available.
func lockdown(allocDir, taskDir string, extra []string) error {
	// landlock not present in the kernel, do not sandbox
	if !landlock.Available() {
		return nil
	}
	paths := []*landlock.Path{
		landlock.DNS(),
		landlock.Certs(),
		landlock.Shared(),
		landlock.Dir("/bin", "rx"),
		landlock.Dir("/usr/bin", "rx"),
		landlock.Dir("/usr/local/bin", "rx"),
		landlock.Dir("/usr/libexec", "rx"),
		landlock.Dir(allocDir, "rwc"),
		landlock.Dir(taskDir, "rwc"),
	}

	for _, p := range extra {
		path, err := landlock.ParsePath(p)
		if err != nil {
			return err
		}
		paths = append(paths, path)
	}
	paths = append(paths, additionalFilesForVCS()...)
	locker := landlock.New(paths...)
	return locker.Lock(landlock.Mandatory)
}

func additionalFilesForVCS() []*landlock.Path {
	const (
		homeSSHDir     = ".ssh"                     // git ssh
		homeKnownHosts = ".ssh/known_hosts"         // git ssh
		etcPasswd      = "/etc/passwd"              // git ssh
		etcKnownHosts  = "/etc/ssh/ssh_known_hosts" // git ssh
		gitGlobalFile  = "/etc/gitconfig"           // https://git-scm.com/docs/git-config#SCOPES
		hgGlobalFile   = "/etc/mercurial/hgrc"      // https://www.mercurial-scm.org/doc/hgrc.5.html#files
		hgGlobalDir    = "/etc/mercurial/hgrc.d"    // https://www.mercurial-scm.org/doc/hgrc.5.html#files
		urandom        = "/dev/urandom"             // git
	)
	return filesForVCS(
		homeSSHDir,
		homeKnownHosts,
		etcPasswd,
		etcKnownHosts,
		gitGlobalFile,
		hgGlobalFile,
		hgGlobalDir,
		urandom,
	)
}

func filesForVCS(
	homeSSHDir,
	homeKnownHosts,
	etcPasswd,
	etcKnownHosts,
	gitGlobalFile,
	hgGlobalFile,
	hgGlobalDir,
	urandom string) []*landlock.Path {

	// omit ssh if there is no home directory
	home := findHomeDir()
	homeSSHDir = filepath.Join(home, homeSSHDir)
	homeKnownHosts = filepath.Join(home, homeKnownHosts)

	// detect if p exists
	exists := func(p string) bool {
		_, err := os.Stat(p)
		return err == nil
	}

	result := make([]*landlock.Path, 0, 6)
	if exists(homeSSHDir) {
		result = append(result, landlock.Dir(homeSSHDir, "r"))
	}
	if exists(homeKnownHosts) {
		result = append(result, landlock.File(homeKnownHosts, "rw"))
	}
	if exists(etcPasswd) {
		result = append(result, landlock.File(etcPasswd, "r"))
	}
	if exists(etcKnownHosts) {
		result = append(result, landlock.File(etcKnownHosts, "r"))
	}
	if exists(gitGlobalFile) {
		result = append(result, landlock.File(gitGlobalFile, "r"))
	}
	if exists(hgGlobalFile) {
		result = append(result, landlock.File(hgGlobalFile, "r"))
	}
	if exists(hgGlobalDir) {
		result = append(result, landlock.Dir(hgGlobalDir, "r"))
	}
	if exists(urandom) {
		result = append(result, landlock.File(urandom, "r"))
	}
	return result
}