2019-10-06 09:23:10

by Laurent Bigonville

[permalink] [raw]
Subject: systemd --user for GDM started as unconfined_t instead of xdm_t

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 <[email protected]>
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 <[email protected]>

Was that expected and/or wanted?

Kind regards,

Laurent Bigonville


2019-10-06 09:49:34

by Dominick Grift

[permalink] [raw]
Subject: Re: systemd --user for GDM started as unconfined_t instead of xdm_t

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 <[email protected]>
> 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 <[email protected]>
>
> 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)

>
> Kind regards,
>
> Laurent Bigonville
>

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2019-10-06 10:01:42

by Dominick Grift

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

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 <[email protected]>
> > 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.PeBenit[email protected]>
> >
> > 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
> >
>
> --
> Key fingerprint = 5F4D 3CDB D3F8 3652 FBD8 02D5 3B6C 5F1D 2C7B 6B02
> https://sks-keyservers.net/pks/lookup?op=get&search=0x3B6C5F1D2C7B6B02
> Dominick Grift



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2019-10-06 10:14:44

by Dominick Grift

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

On Sun, Oct 06, 2019 at 12:01:25PM +0200, Dominick Grift wrote:
> 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 <[email protected]>
> > > 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 <[email protected]>
> > >
> > > 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.


I wonder what you happen if you would (would gdm still work?):

sudo systemctl mask [email protected]$(id -u gdm).service [email protected]$(id -u gdm).service

>
> >
> > >
> > > Kind regards,
> > >
> > > Laurent Bigonville
> > >
> >
> > --
> > Key fingerprint = 5F4D 3CDB D3F8 3652 FBD8 02D5 3B6C 5F1D 2C7B 6B02
> > https://sks-keyservers.net/pks/lookup?op=get&search=0x3B6C5F1D2C7B6B02
> > Dominick Grift
>
>
>
> --
> Key fingerprint = 5F4D 3CDB D3F8 3652 FBD8 02D5 3B6C 5F1D 2C7B 6B02
> https://sks-keyservers.net/pks/lookup?op=get&search=0x3B6C5F1D2C7B6B02
> Dominick Grift



--
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Dominick Grift


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