Ansible can use existing privilege escalation systems to allow a user to execute tasks as another.
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.
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, dzdo (Centrify), and others.
Become vars & directives are independent, i.e. setting become_user does not set become.
These can be set from play to task level, but are overridden by connection variables as they can be host specific.
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
- name: Ensure the httpd service is running service: name: httpd state: started become: true
To run a command as the
- 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'
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.
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
|ask for privilege escalation password, does not imply become will be used|
|--become, -b||run operations with become (no password implied)|
|privilege escalation method to use (default=sudo), valid choices: [ sudo | su | pbrun | pfexec | doas | dzdo | ksu ]|
|run operations as this user (default=root), does not imply –become/-b|
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.
Although privilege escalation is mostly intuitive, there are a few limitations on how it works. Users should be aware of these to avoid surprises.
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_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
In Ansible 2.1, this window is further narrowed: If the connection
is made as a privileged user (root), then Ansible 2.1 and above will use
chown to set the file’s owner to the unprivileged user being switched to.
This means both the user making the connection and the user being switched
become must be unprivileged in order to trigger this problem.
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:
PATHthen 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.
becomeroot 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
becometo an unprivileged account.
Although the Solaris ZFS filesystem has filesystem ACLs, the ACLs
are not POSIX.1e filesystem acls (they are NFSv4 ACLs instead). Ansible
cannot use these ACLs to manage its temp file permissions so you may have
to resort to
allow_world_readable_tmpfiles if the remote machines use ZFS.
Changed in version 2.1.
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.
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).
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).
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.