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OS Login Guest Environment for Google Compute Engine

This repository contains the system components responsible for providing Google Cloud OS Login features on Google Compute Engine instances.

Table of Contents

Overview

The OS Login Guest Environment consists of the following main components:

  • Authorized Keys Command which provides SSH keys from the user's OS Login profile to sshd for authenticating users at login.
  • NSS Modules which provide support for making OS Login user and group information available to the system, using NSS (Name Service Switch) functionality.
  • PAM Modules which provide authorization (and authentication if two-factor support is enabled) support allowing the system to use Google Cloud IAM permissions to control the ability to log into an instance or to perform operations as root (via sudo).

In addition to the main components, there are also the following utilities:

  • google_oslogin_nss_cache is a utility for updating the local user and group cache.
  • selinux contains SELinux policy definition files and a compiled policy package for configuring SELinux to support OS Login.

The packaging directory also contains files used to generate .deb and .rpm packages for the OS Login components.

Components

Authorized Keys Command

The google_authorized_keys binary is designed to be used with the sshd AuthorizedKeysCommand option in sshd_config(5). It does the following:

  • Reads the user's profile information from the metadata server:
    http://metadata.google.internal/computeMetadata/v1/oslogin/users?username=<username>
    
  • Checks to make sure that the user is authorized to log in:
    http://metadata.google.internal/computeMetadata/v1/oslogin/authorize?email=<user_email>&policy=login
    
  • If the check is successful, returns the SSH keys associated with the user for use by sshd. Otherwise, exits with an error code.

NSS Modules

libnss_oslogin.so and libnss_cache_oslogin.so are NSS service modules which make OS Login users and groups available for use on the local system. The module is activated by adding oslogin and cache_oslogin entries for services in nsswitch.conf(5).

  • To return a list of all users, the NSS module queries:
    http://metadata.google.internal/computeMetadata/v1/oslogin/users?pagesize=<pagesize>
    
  • To look up a user by username, the NSS module queries:
    http://metadata.google.internal/computeMetadata/v1/oslogin/users?username=<username>
    
  • To look up a user by UID, the NSS module queries:
    http://metadata.google.internal/computeMetadata/v1/oslogin/users?uid=<uid>
    

PAM Modules

pam_oslogin_login.so is a PAM module which determines whether a given user is allowed to SSH into an instance.

It is activated by adding an entry for the account group to the PAM service config for sshd as:

account requisite pam_oslogin_login.so

This module:

  • Retrieves the user's profile information from the metadata server:
    http://metadata.google.internal/computeMetadata/v1/oslogin/users?username=<username>
    
  • If the user does not have OS Login profile information it is passed on to the system authentication modules to be processed as a local user.
  • Otherwise, the module confirms whether the user has permissions to SSH into the instance:
    http://metadata.google.internal/computeMetadata/v1/oslogin/authorize?email=<user_email>&policy=login
    
  • If the user is authorized, PAM returns a success message and SSH can proceed. Otherwise, PAM returns a denied message and the SSH check will fail.

pam_oslogin_admin.so is a PAM module which determines whether a given user should have admin (sudo) permissions on the instance.

It is activated by adding an entry for the account group to the PAM service config for sshd config as:

account optional pam_oslogin_admin.so

This module:

  • Retrieves the user's profile information from the metadata server.
    http://metadata.google.internal/computeMetadata/v1/oslogin/users?username=<username>
    
  • If the user is not an OS Login user (a local user account), the module returns success.
  • Otherwise, the module determines if the user has admin permissions:
    http://metadata.google.internal/computeMetadata/v1/oslogin/authorize?email=<user_email>&policy=adminLogin
    
  • If the user is authorized as an admin, a file with the username is added to /var/google-sudoers.d/. The file gives the user sudo privileges.
  • If the authorization check fails for admin permissions, the file is removed from /var/google-sudoers.d/ if it exists.

Utilities

SELinux Policy

The selinux directory contains .te (type enforcement) and .fc (file context) files used by SELinux to give the OS Login features the appropriate SELinux permissions. These are compiled using checkmodule and semodule_package to create an policy package oslogin.pp.

Source Packages

There is currently support for creating packages for the following distros:

  • Debian 9
  • CentOS/RHEL 6
  • CentOS/RHEL 7

Files for these packages are in the packaging/ directory.