Re: systemd --user for GDM started as unconfined_t instead of xdm_t

[Dominick Grift <dac.override@gmail.com>]

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On Sun, Oct 06, 2019 at 11:41:56AM +0200, Dominick Grift wrote:
> On Sun, Oct 06, 2019 at 11:22:00AM +0200, Laurent Bigonville wrote:
> > Hello,
> > 
> > I discovered today that the GDM own processes are started as unconfined_t
> > instead of xdm_t because systemd --user process itself is started in that
> > context.
> > 
> > This is probably related to:
> > 
> > commit da156aea1e89a6ff6025be7e50c9c8173e5a6dcf
> > Author: Chris PeBenito <Christopher.PeBenito@microsoft.com>
> > Date:   Fri Apr 19 11:50:59 2019 -0400
> > 
> >     systemd: Add initial policy for systemd --user.
> > 
> >     This is just a start; it does not cover all uses.
> > 
> >     Signed-off-by: Chris PeBenito <Christopher.PeBenito@microsoft.com>
> > 
> > Was that expected and/or wanted?
> 
> It just means that gdm hooks into pam, and since 1. your __default__ id is set to unconfined_u and 2. you do not have a private id for gdm (and gnome-initial-setup) systemd will start gdm's systemd --user instance with unconfined_u:unconfined_r:unconfined_t.
> 
> One (ugly but arguable less ugly than the alternative) solution is to create a "xdm_u" and allow systemd to run a systemd --user instance on behalf of gdm with "xdm_u:system_r:xdm_t"
> 
> That way you can tell selinux that gdm's systemd --user instance should never transition out of xdm_u:system_r:xdm_t
> 
> echo "system_r:init_t:s0 system_r:xdm_t:s0" > /etc/selinux/TYPE/contexts/users/xdm_u
> 
> Then all processes in the gdm session should stay in xdm_t (but some processes will be associate with xdm_u and other with system_u).
> 
> You would probably also want to add to semanage.conf:
> 
> ignoredirs = /var/lib/gdm;/run/gnome-initial-setup
> 
> and make sure that selinux does not relabel /run/user/$(id -u gdm)

Just to clarify. The patch you reference is not responsible for the uglyness that is GDM/Gnome
Without this patch, the systemd --user instance of GDM (and any other users) would run in "init_t". This is obviously also not desirable.

> 
> > 
> > Kind regards,
> > 
> > Laurent Bigonville
> > 
> 
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> Dominick Grift



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Dominick Grift

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