Support a user-session mode for authenticating

[Michael Terry]

[Impact]
Ubuntu phone development requires in session greeter functionality to perform suitably. This functionality does not exist in current versions of LightDM.

[Test Case]
1. Install a session that supports in session greeter
2. Log into that session
3. Activate greeter functionality

Expected result:
It works

Observed result:
The functionality is not implemented.

[Regression Potential]
Some risk of changing existing behaviour, reduced by regression tests still passing. Functionality is only enabled for sessions that opt-in to this behaviour so additional security risks is limited to new sessions.

Original description:

We talked about this in person in Prague.

Ideally a greeter could run as in the user's session and act as a lockscreen for the user as well as a way to authenticate/log-in as other users.

This would let us avoid running a whole other session for the greeter on the phone, which is memory intensive.

It might be easiest for unity8 if that API was still just liblightdm, just running in a special mode. But I'm not picky on how it's done.

There may be security questions around this. I asked Jamie and Tyler about it, I'll post any concerns from them.

[Robert Ancell (robert-ancell)]

This is now in lp:lightdm. To make an in-session greeter you need to set X-LightDM-Allow-Greeter=true in the session .desktop file. Then you can run a greeter using liblightdm from inside that session.

Only one greeter is allowed at once. Any process from the logged in user can connect (so there is a denial of service risk if the correct process doesn't get the socket first or loses it).

The socket is called /var/run/lightdm/<user>/greeter-socket. For security purposes it's probably worth locking this down with AppArmor rules.

[Launchpad Janitor (janitor)]

This bug was fixed in the package lightdm - 1.19.2-0ubuntu1

---------------
lightdm (1.19.2-0ubuntu1) yakkety; urgency=medium

  * New upstream release:
    * Add support for greeters running inside sessions. This is enabled by
      setting X-LightDM-Allow-Greeter inside the session .desktop file.
      The session can then use liblightdm to connect one greeter to the
      daemon. The communication is done using a socket
      (/var/run/lightdm/<user>/greeter-socket) that is accessible to any
      process run by that user. Consider controlling access to this socket
      using a MAC system such as AppArmor. (LP: #1582242)
    * Report errors for all liblightdm methods. This will require existing
      greeters to update their API usage. The ABI is unchanged.
    * Handle EAGAIN correctly when daemons communicate with the daemon.
    * Drop support for mir-container sessions - no-one ever used these.
  * debian/liblightdm-gobject-1-0.symbols:
    - Updated

 -- Robert Ancell <email address hidden> Wed, 29 Jun 2016 15:29:17 +1200

[Timo Aaltonen (tjaalton)]

Hello Michael, or anyone else affected,

Accepted lightdm into xenial-proposed. The package will build now and be available at https://launchpad.net/ubuntu/+source/lightdm/1.18.3-0ubuntu1 in a few hours, and then in the -proposed repository.

Please help us by testing this new package. See https://wiki.ubuntu.com/Testing/EnableProposed for documentation how to enable and use -proposed. Your feedback will aid us getting this update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug, mentioning the version of the package you tested, and change the tag from verification-needed to verification-done. If it does not fix the bug for you, please add a comment stating that, and change the tag to verification-failed. In either case, details of your testing will help us make a better decision.

Further information regarding the verification process can be found at https://wiki.ubuntu.com/QATeam/PerformingSRUVerification . Thank you in advance!

[Michael Terry (mterry)]

Can we not backport 1.19.3? That has a few crashing issues with in-session greeters. There are fixes in trunk though, so 1.19.4 would be better.

[Robert Ancell (robert-ancell)]

1.18.3 should contain all those changes backported...

[Robert Ancell (robert-ancell)]

Michael - can you confirm if this is fixed in the SRU?

[Michael Terry (mterry)]

Oh, I think I remember confirming it at one point yeah.

[Launchpad Janitor (janitor)]

This bug was fixed in the package lightdm - 1.18.3-0ubuntu1

---------------
lightdm (1.18.3-0ubuntu1) xenial; urgency=medium

  * New upstream release:
    - Fix error that caused stdin to be closed. This seems to have lots of bad
      side effects (greeter not working correctly, multi-seat failing).
    - Add support for greeters running inside sessions. This is enabled by
      setting X-LightDM-Allow-Greeter inside the session .desktop file.
      The session can then use liblightdm to connect one greeter to the
      daemon. The communication is done using a socket
      (/var/run/lightdm/<user>/greeter-socket) that is accessible to any
      process run by that user. Consider controlling access to this socket
      using a MAC system such as AppArmor. (LP: #1582242)
    - Fix various memory management issues exposed by the use of in-session
      greeters.
  * debian/patches/lp1605117.patch:
    - Applied upstream

 -- Robert Ancell <email address hidden> Fri, 12 Aug 2016 11:37:43 +1200

[Martin Pitt (pitti)]

The verification of the Stable Release Update for lightdm has completed successfully and the package has now been released to -updates. Subsequently, the Ubuntu Stable Release Updates Team is being unsubscribed and will not receive messages about this bug report. In the event that you encounter a regression using the package from -updates please report a new bug using ubuntu-bug and tag the bug report regression-update so we can easily find any regressions.