Become (Privilege Escalation)

Ansible can use existing privilege escalation systems to allow a user to execute tasks as another.

• Become
  • Directives
  • Connection variables
  • Command line options
  • For those from Pre 1.9 , sudo and su still work!
  • Limitations
    • Becoming an Unprivileged User
    • Connection Plugin Support
    • Only one method may be enabled per host
    • Can’t limit escalation to certain commands

Become

Ansible allows you to ‘become’ another user, different from the user that logged into the machine (remote user). This is done using existing privilege escalation tools, which you probably already use or have configured, like sudo, su, pfexec, doas, pbrun, dzdo, ksu and others.

Directives

These can be set from play to task level, but are overridden by connection variables as they can be host specific.

become

set to ‘true’/’yes’ to activate privilege escalation.

become_user

set to user with desired privileges — the user you ‘become’, NOT the user you login as. Does NOT imply become: yes, to allow it to be set at host level.

become_method

(at play or task level) overrides the default method set in ansible.cfg, set to sudo/su/pbrun/pfexec/doas/dzdo/ksu

become_flags

(at play or task level) permit to use specific flags for the tasks or role. One common use is to change user to nobody when the shell is set to no login. Added in Ansible 2.2.

For example, to manage a system service (which requires root privileges) when connected as a non-root user (this takes advantage of the fact that the default value of become_user is root):

- name: Ensure the httpd service is running
  service:
    name: httpd
    state: started
  become: true

To run a command as the apache user:

- name: Run a command as the apache user
  command: somecommand
  become: true
  become_user: apache

To do something as the nobody user when the shell is nologin:

- name: Run a command as nobody
  command: somecommand
  become: true
  become_method: su
  become_user: nobody
  become_flags: '-s /bin/sh'

Connection variables

Each allows you to set an option per group and/or host, these are normally defined in inventory but can be used as normal variables.

ansible_become

equivalent of the become directive, decides if privilege escalation is used or not.

ansible_become_method

allows to set privilege escalation method

ansible_become_user

allows to set the user you become through privilege escalation, does not imply ansible_become: True

ansible_become_pass

allows you to set the privilege escalation password

For example, if you want to run all tasks as root on a server named webserver, but you can only connect as the manager user, you could use an inventory entry like this:

webserver ansible_user=manager ansible_become=true

Command line options

--ask-become-pass, -K
	ask for privilege escalation password, does not imply become will be used
--become, -b	run operations with become (no password implied)
--become-method=BECOME_METHOD
	privilege escalation method to use (default=sudo), valid choices: [ sudo | su | pbrun | pfexec | doas | dzdo | ksu ]
--become-user=BECOME_USER
	run operations as this user (default=root), does not imply –become/-b

For those from Pre 1.9 , sudo and su still work!

For those using old playbooks will not need to be changed, even though they are deprecated, sudo and su directives, variables and options will continue to work. It is recommended to move to become as they may be retired at one point. You cannot mix directives on the same object (become and sudo) though, Ansible will complain if you try to.

Become will default to using the old sudo/su configs and variables if they exist, but will override them if you specify any of the new ones.

Limitations

Although privilege escalation is mostly intuitive, there are a few limitations on how it works. Users should be aware of these to avoid surprises.

Becoming an Unprivileged User

Ansible 2.0.x and below has a limitation with regards to becoming an unprivileged user that can be a security risk if users are not aware of it. Ansible modules are executed on the remote machine by first substituting the parameters into the module file, then copying the file to the remote machine, and finally executing it there.

Everything is fine if the module file is executed without using become, when the become_user is root, or when the connection to the remote machine is made as root. In these cases the module file is created with permissions that only allow reading by the user and root.

The problem occurs when the become_user is an unprivileged user. Ansible 2.0.x and below make the module file world readable in this case, as the module file is written as the user that Ansible connects as, but the file needs to be readable by the user Ansible is set to become.

If any of the parameters passed to the module are sensitive in nature, then those pieces of data are located in a world readable module file for the duration of the Ansible module execution. Once the module is done executing, Ansible will delete the temporary file. If you trust the client machines then there’s no problem here. If you do not trust the client machines then this is a potential danger.

Ways to resolve this include:

• Use pipelining. When pipelining is enabled, Ansible doesn’t save the module to a temporary file on the client. Instead it pipes the module to the remote python interpreter’s stdin. Pipelining does not work for non-python modules.
• (Available in Ansible 2.1) Install POSIX.1e filesystem acl support on the managed host. If the temporary directory on the remote host is mounted with POSIX acls enabled and the setfacl tool is in the remote PATH then Ansible will use POSIX acls to share the module file with the second unprivileged user instead of having to make the file readable by everyone.
• Don’t perform an action on the remote machine by becoming an unprivileged user. Temporary files are protected by UNIX file permissions when you become root or do not use become. In Ansible 2.1 and above, UNIX file permissions are also secure if you make the connection to the managed machine as root and then use become to an unprivileged account.

In addition to the additional means of doing this securely, Ansible 2.1 also makes it harder to unknowingly do this insecurely. Whereas in Ansible 2.0.x and below, Ansible will silently allow the insecure behaviour if it was unable to find another way to share the files with the unprivileged user, in Ansible 2.1 and above Ansible defaults to issuing an error if it can’t do this securely. If you can’t make any of the changes above to resolve the problem, and you decide that the machine you’re running on is secure enough for the modules you want to run there to be world readable, you can turn on allow_world_readable_tmpfiles in the ansible.cfg file. Setting allow_world_readable_tmpfiles will change this from an error into a warning and allow the task to run as it did prior to 2.1.

Connection Plugin Support

Privilege escalation methods must also be supported by the connection plugin used. Most connection plugins will warn if they do not support become. Some will just ignore it as they always run as root (jail, chroot, etc).

Only one method may be enabled per host

Methods cannot be chained. You cannot use sudo /bin/su - to become a user, you need to have privileges to run the command as that user in sudo or be able to su directly to it (the same for pbrun, pfexec or other supported methods).

Can’t limit escalation to certain commands

Privilege escalation permissions have to be general. Ansible does not always use a specific command to do something but runs modules (code) from a temporary file name which changes every time. If you have ‘/sbin/service’ or ‘/bin/chmod’ as the allowed commands this will fail with ansible as those paths won’t match with the temporary file that ansible creates to run the module.