RFD 45 - RBAC where conditions for active sessions list/read

What

Manage access to active sessions (resource kind ssh_session) by RBAC where conditions, in the same manner as the RFD 44 RBAC where conditions for session recordings list/read provides access management for session recordings (resource kind session).

These deny checks are to be employed on top of the new RBAC rules for listing and joining sessions introduced in RFD 43. This means that the user must pass both the resource checks introduced in this RFD and the RBAC join_policy checks from RFD 43 in order to join a session.

Why

To be able to restrict access of certain users to only a subset of active sessions, notably only their own active sessions.

Details

Work around implicit allow rule

Unlike session, the ssh_session kind is referred to by an implicit rule granting unrestricted list/read privileges to all users:

	types.NewRule(types.KindSSHSession, RO()),

Adding a where section to an explicit allow rule for ssh_session would therefore take no effect. To restrict access to active sessions, one has to add (the negation of) the desired condition to a deny rule, as those are applied earlier than allow rules:

spec:
  deny:
    rules:
    - resources: [ssh_session]
      verbs: [list, read]
      where: '!contains(ssh_session.participants, user.metadata.name)'

ssh_session identifier API

The ssh_session identifier exposes the private lib.session.ctxSession struct. ctxSession is a subset of session.Session with the addition of the participants field.

In general, the RBAC contains predicate can only be used to detect the occurrence of a string in a slice of strings. However, the Parties field of session.Session is a slice of more complex Party objects, not strings. To support where conditions as above, a list of usernames is extracted from the original Party slice in order that the usernames be then bound to ssh_session.participants instead.