Become (Privilege Escalation)¶

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

Topics

Become (Privilege Escalation)
- Become

Become ¶

Before 1.9 Ansible mostly allowed the use of sudo and a limited use of su to allow a login/remote user to become a different user and execute tasks, create resources with the 2nd user’s permissions. As of 1.9 become supersedes the old sudo/su, while still being backwards compatible. This new system also makes it easier to add other privilege escalation tools like pbrun (Powerbroker), pfexec and others.

New directives ¶

become: equivalent to adding sudo: or su: to a play or task, set to ‘true’/’yes’ to activate privilege escalation
become_user: equivalent to adding ‘sudo_user:’ or ‘su_user:’ to a play or task, set to user with desired privileges
become_method: at play or task level overrides the default method set in ansible.cfg, set to ‘sudo’/’su’/’pbrun’/’pfexec’/’doas’

New ansible_ variables ¶

Each allows you to set an option per group and/or host

ansible_become: equivalent to ansible_sudo or ansible_su, allows to force privilege escalation
ansible_become_method: allows to set privilege escalation method
ansible_become_user: equivalent to ansible_sudo_user or ansible_su_user, allows to set the user you become through privilege escalation
ansible_become_pass: equivalent to ansible_sudo_pass or ansible_su_pass, allows you to set the privilege escalation password

New command line options ¶

`--ask-become-pass`
	ask for privilege escalation password

–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 ]
`--become-user=BECOME_USER`
	run operations as this user (default=root)

sudo and su still work!¶

Old playbooks will not need to be changed, even though they are deprecated, sudo and su directives will continue to work though it is recommended to move to become as they may be retired at one point. You cannot mix directives on the same object 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 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. 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 then the module file is created with permissions that only allow reading by the user and root.

If the become user is an unprivileged user and then Ansible has no choice but to make the module file world readable as there’s no other way for the user Ansible connects as to save the file so that the user that we’re becoming can read it.

If any of the parameters passed to the module are sensitive in nature then those pieces of data are readable by reading the 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.
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.

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.