Guest logins, (was Re: [RFC] Merging LightDM into KDE Workspaces (forwarded from plasma-devel))

Thomas Lübking thomas.luebking at
Wed Aug 29 21:17:00 BST 2012

Am 29.08.2012, 03:21 Uhr, schrieb Harald Sitter <sitter at>:

> On Wed, Aug 29, 2012 at 2:59 AM, Thomas Lübking
> <thomas.luebking at> wrote:
>> but I'm willing to
>> be enlightened about the striking advances of this integration.

Thanks for the link.

Reads like they're looking for

useradd -md /tmp/ubuntu-guest ubuntu-guest
usermod -L ubuntu-guest

and a guest-session script

---- guest-session---
if [ $(id -u) != 0 ]; then
    logger "`whoami` allowed guest session"
    /usr/lib/kde4/libexec/kscreenlocker --forcelock &
    sudo/pkexec guest-session
    logger "`whoami` permitted guest session ended"

su ubuntu-guest
export DISPLAY=:1 #todo - not hardcode :1
X :1 & sleep 1 & ck-launch-session -display :1 startkde #nobody really  
uses unity
#or so - X does not auto-end in this just written down script.


This is it?

Brings us to the questioned DM integration requirement.
The only part remotely suggesting a special integration with a DM seems:

     "Thus we will extend gdm to allow spawning a new X server for the  
special :guest user (which is not a valid PAM user name and thus cannot  
collide with any real user)."

What is no way a requirement but a choice.
There is no reason to control access this way (ie. prevent remote logins  
on that user or whatever - pam_access does that and locked users can't  
login this way at all) and there's no reason to create an invalid PAM user  
account to prevent collisions either (one can pre-allocate one or create  
one guest-<rnd> and check that against /etc/passwd until there's no clash)  
and, actually, prevents two coincident guest logins.

Just locking the account and sudo/polkit su to it would ensure to only  
allow such authenticated "login".

So maybe i'm just stupid, but my question frankly remains:
why should one turn the DM into an AM to add a locked down guest account,  
if the same can be achieved by a mini script and maybe a polkit rule?

The functionality can be provided aside the DM for those who really want  
it - the particular thing is called "LightDM" and it's not even "only" a  



Before mentioned:
The only aspect are guest logins from the DM as *suggested* by

     ""An existing user must authenticate the start of a guest session"  
should be optional, so that someone can use the computer without anyone  
else being present."

What however completely contradicts (the very much true)

     "This design shouldn't aim for ultimate security. In the use cases,  
the guest users are somewhat trusted and also observed while they work on  
the box. We do not design a solution that can resist half an hour of  
unobserved tampering. In particular, this is not strong enough to be a  
fully secure kiosk solution (which is why we require authentication from  
an existing user)."

And also invalidates the

     "Common practice for many people is to create a "guest" user account  
with a trivial or empty or empty password, but this is a potential  
security threat."

rationale, because one just added a permanent pwdless account (ideally at  
least not for remote logins)

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