The polkit interface

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The polkit interface provides daemons with the permission to use the polkit authorisation manager (polkitd) to make access control decisions for requests from unprivileged clients.

Interface documentation:

See Interface management and Supported interfaces for further details on how interfaces are used.

Developer details

Auto-connect: no
Super-privileged: yes

Attributes:

  • action-prefix (plug, mandatory): indicate that all actions published by the snap are equal to the action prefix or match ${action-prefix}.* .

To perform polkit authorisation checks, a daemon needs to do two things:

  1. Install a .policy file to $SNAP/meta/polkit/${plug_name}.*.policy describing the actions it will use (codifying the type of administrative access a user might be granted). Snapd will install the policy file when the plug is connected.
  2. Before performing administrative work on behalf of a client app, make a CheckAuthorization D-Bus call to polkitd to ask if they have access. The D-Bus call passes a string action ID describing the access, and a “subject” struct describing the client application.

There are two primary ways a daemon can describe the subject of the check:

  1. For D-Bus daemons they can use a system-bus-name subject, sending the unique bus name of the client app.
  2. For non-D-Bus daemons, they can use a unix-process subject, sending the process ID (as retrieved through SO_PEERCRED or SCM_CREDENTIALS).

See Proposal: add polkit and polkit-agent interfaces to snapd for the original interface proposal and reasoning.

Code examples

plugs:
 polkit:
  action-prefix: org.example.foo

apps:
 app:
  command: foo
  plugs: [polkit]

The test code can be found in the snapd repository: snapd/interfaces/builtin/polkit_test.go at master · canonical/snapd · GitHub

The source code for the interface is in the snapd repository: snapd/interfaces/builtin/polkit.go at master · canonical/snapd · GitHub