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Date:   Sun, 6 Oct 2019 11:41:56 +0200
From:   Dominick Grift <dac.override@gmail.com>
To:     Laurent Bigonville <bigon@debian.org>
Cc:     selinux-refpolicy@vger.kernel.org
Subject: Re: systemd --user for GDM started as unconfined_t instead of xdm_t
Message-ID: <20191006094156.GB469820@brutus.lan>
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On Sun, Oct 06, 2019 at 11:22:00AM +0200, Laurent Bigonville wrote:
> Hello,
>=20
> 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.
>=20
> This is probably related to:
>=20
> commit da156aea1e89a6ff6025be7e50c9c8173e5a6dcf
> Author: Chris PeBenito <Christopher.PeBenito@microsoft.com>
> Date:=A0=A0 Fri Apr 19 11:50:59 2019 -0400
>=20
> =A0=A0=A0 systemd: Add initial policy for systemd --user.
>=20
> =A0=A0=A0 This is just a start; it does not cover all uses.
>=20
> =A0=A0=A0 Signed-off-by: Chris PeBenito <Christopher.PeBenito@microsoft.c=
om>
>=20
> 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 unconf=
ined_u:unconfined_r:unconfined_t.

One (ugly but arguable less ugly than the alternative) solution is to creat=
e 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 nev=
er transition out of xdm_u:system_r:xdm_t

echo "system_r:init_t:s0 system_r:xdm_t:s0" > /etc/selinux/TYPE/contexts/us=
ers/xdm_u

Then all processes in the gdm session should stay in xdm_t (but some proces=
ses will be associate with xdm_u and other with system_u).

You would probably also want to add to semanage.conf:

ignoredirs =3D /var/lib/gdm;/run/gnome-initial-setup

and make sure that selinux does not relabel /run/user/$(id -u gdm)

>=20
> Kind regards,
>=20
> Laurent Bigonville
>=20

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Dominick Grift

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