Since the fix for CVE-2023-3899 landed (RHELPLAN-166172 for 9.3.0), registering a system through the GUI on the logged in session doesn't work:

Reproduction steps:

• Install from the DVD installer without registering
• On first login, see a notification, click on it to go to GNOME Settings About page
• Scroll down to "Subscription", click on that to get the registration dialog
• Try to register - get an error

[ I haven't tested the above, but have reproduced with 9.2 by installing a system, fulling updating it, then unregistering it]

The problem is that the way that gnome-settings-daemon registers:

1) Ask RHSM to start a private root-only accessible bus
2) Run a helper (gsdk-subman-helper) as root to access the bus and register
3) Ask RHSM to stop the bus

The fix here could either be:
A) In subscription manager, allow (targeted) non-root access to this start/stop interface again
B) In gnome-settings-daemon, move the start/stop bus operation to gsd-subman-helper

A) is simpler - no new code, just config changes - if we can verify that the we're sure there is no harm from allowing non-root users to start the bus.