How can a file manager mount a drive without root?

Users operating at the console of a graphical workstation have noted that several programs can be executed without apparently needing root authentication nor a password such as reboot. This process involves the clever use of the SUID program /usr/sbin/userhelper applied in a broader context than originally designed.

The graphical user executes an intermediary aliased program /usr/bin/consolehelper which authorizes actions based on a specific PAM (Programmable Authentication Modules) configuration and then sends the command off to a SUID program to execute the user program with privileges. If the user does not have appropriate authorization, then the requested program is executed under the users’ Linux environment.

As currently deployed, the needed PAM configuration file for reboot contains checks for the user to be logged in at the console or be currently running under the root environment to inhibit password requests.

It uses udisks. (FUSE is used in the case of network filesystems however). Command-line interfaces to udisks are available for your experimentation. In recent versions it comes with the command-line interface udisksctl.

udisksd runs as root, and accepts the user requests using D-Bus.

udisksd uses PolicyKit PolKit to decide which requests are permitted. Some may require extra privilege e.g. formatting internal drives. This can involve an extra authentication step - similar to Windows UAC prompt, or sudo on the command line. I don't know exactly how the extra authentication is co-ordinated.

Requests over D-Bus are currently made using Unix sockets using SCM_CREDENTIALS, which identifies the process making the request.

One of the factors used in decisions is whether the user is logged in locally, or over the network (e.g. ssh). I believe this information is provided by systemd-logind (in conspiracy with pam-systemd).