GNU bug report logs - #35674
GDM auto-login doesn't work

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: "Alex Griffin" <a <at> ajgrf.com>
To: bug-guix <at> gnu.org
Subject: GDM auto-login doesn't work
Date: Fri, 10 May 2019 16:31:27 -0400

[Message part 1 (text/plain, inline)]

After configuring Guix to enable GDM auto-login, I am still presented with a password prompt upon booting my machine. I'm not sure whether it's a bug in Guix or something I'm doing wrong, so I've attached the configuration I'm using just in case.
-- 
Alex Griffin

[system.scm (text/x-scheme, attachment)]

Message #8 received at 35674 <at> debbugs.gnu.org (full text, mbox):

From: Ricardo Wurmus <rekado <at> elephly.net>
To: Alex Griffin <a <at> ajgrf.com>
Cc: 35674 <at> debbugs.gnu.org
Subject: Re: bug#35674: GDM auto-login doesn't work
Date: Fri, 10 May 2019 23:18:59 +0200

Hi Alex,

> After configuring Guix to enable GDM auto-login, I am still presented
> with a password prompt upon booting my machine. I'm not sure whether
> it's a bug in Guix or something I'm doing wrong, so I've attached the
> configuration I'm using just in case.

I have observed the same problem.  It’s not something you’re doing wrong.

--
Ricardo

Message #11 received at 35674 <at> debbugs.gnu.org (full text, mbox):

From: Timothy Sample <samplet <at> ngyro.com>
To: 35674 <at> debbugs.gnu.org
Cc: Ricardo Wurmus <rekado <at> elephly.net>, Alex Griffin <a <at> ajgrf.com>
Subject: Re: bug#35674: GDM auto-login doesn't work
Date: Mon, 13 May 2019 10:24:58 -0400

Hello,

Ricardo Wurmus <rekado <at> elephly.net> writes:

> Hi Alex,
>
>> After configuring Guix to enable GDM auto-login, I am still presented
>> with a password prompt upon booting my machine. I'm not sure whether
>> it's a bug in Guix or something I'm doing wrong, so I've attached the
>> configuration I'm using just in case.
>
> I have observed the same problem.  It’s not something you’re doing wrong.

I noticed recently that GDM stopped reading its configuration file.  I
pushed a430a3501a6d3a565cb78e04a8dbb3ab846ec5fc, which fixes that
problem, but unfortunately does not fix auto-login.

If I turn on debugging output, I can see that the issue has to do with
the way PAM is configured.  Digging a little deeper, I found that our
auto-login PAM service is

    auth [success=ok default=1] pam_gdm.so
    auth sufficient pam_permit.so

What this means is that if “pam_gdm.so” is not successful, the
“pam_permit.so” line will be skipped, and auto-login will not work.  The
“pam_gdm.so” module does some sort of cached password lookup using the
“keyutils” library (presumably for an encrypted home directory).  Our
build of GDM does not support this (we don’t include “keyutils” in its
inputs), so the module never succeeds.  As a result, auto-login fails.

It looks like this particular way of doing things was cribbed from Red
Hat, where the module that gets skipped by “default=1” is
“pam_gnome_keyring.so” (and not “pam_permit.so”).  Other distros simply
mark it as optional.  I suggest we either omit the first rule, since it
will never do anything anyway, or follow other distros and change its
control field to “optional”.

My experience with PAM is limited, so I would appreciate a second
opinion before committing anything.


-- Tim

Message #14 received at 35674 <at> debbugs.gnu.org (full text, mbox):

From: "Alex Griffin" <a <at> ajgrf.com>
To: "Timothy Sample" <samplet <at> ngyro.com>, 35674 <at> debbugs.gnu.org
Cc: Ricardo Wurmus <rekado <at> elephly.net>
Subject: Re: bug#35674: GDM auto-login doesn't work
Date: Fri, 01 Nov 2019 16:02:41 +0000

[Message part 1 (text/plain, inline)]

On Mon, May 13, 2019, at 2:24 PM, Timothy Sample wrote:
> If I turn on debugging output, I can see that the issue has to do with
> the way PAM is configured.  Digging a little deeper, I found that our
> auto-login PAM service is
> 
>     auth [success=ok default=1] pam_gdm.so
>     auth sufficient pam_permit.so
> 
> What this means is that if “pam_gdm.so” is not successful, the
> “pam_permit.so” line will be skipped, and auto-login will not work.  The
> “pam_gdm.so” module does some sort of cached password lookup using the
> “keyutils” library (presumably for an encrypted home directory).  Our
> build of GDM does not support this (we don’t include “keyutils” in its
> inputs), so the module never succeeds.  As a result, auto-login fails.

I tried adding keyutils to the GDM package and it made no difference. I confirmed that it linked correctly, but didn't look into it further.

> It looks like this particular way of doing things was cribbed from Red
> Hat, where the module that gets skipped by “default=1” is
> “pam_gnome_keyring.so” (and not “pam_permit.so”).  Other distros simply
> mark it as optional.  I suggest we either omit the first rule, since it
> will never do anything anyway, or follow other distros and change its
> control field to “optional”.

I can confirm that changing the control value to "optional" satisfies PAM, but I ran into another problem. GDM continually crashes and restarts about 40 times once autologin is enabled. If I switch to another tty and run `herd restart xorg-server`, then GDM starts fine and automatically logs me in.

I've attached a file with a bunch of log messages. Not full log files, but snippets from a bunch of logs that I thought looked relevant.

> My experience with PAM is limited, so I would appreciate a second
> opinion before committing anything.

This bug was my introduction to PAM, so perhaps my opinion isn't very reassuring, but changing pam_gdm.so to optional seems harmless in this case. Even if it somehow screws something up, it will only affect people who went out of their way to make their system less secure by enabling auto-login.

-- 
Alex Griffin

[gdm_errors.txt (text/plain, attachment)]

Message #19 received at 35674 <at> debbugs.gnu.org (full text, mbox):

From: Alex Griffin <a <at> ajgrf.com>
To: "guix-patches <at> gnu.org" <guix-patches <at> gnu.org>
Cc: "35674 <at> debbugs.gnu.org" <35674 <at> debbugs.gnu.org>,
 "40411 <at> debbugs.gnu.org" <40411 <at> debbugs.gnu.org>
Subject: [PATCH] services: Fix gdm-autologin pam service.
Date: Sun, 14 Jun 2020 23:24:56 +0000

[Message part 1 (text/plain, inline)]

This patch fixes GDM's auto-login feature.

See the discussions in #35674 and #40411. It works for me, but I don't want to just commit it because I don't really understand PAM. (Then again, who does?)

--
Alex Griffin

[0001-services-Fix-gdm-autologin-pam-service.patch (text/x-patch, attachment)]

Message #22 received at 35674 <at> debbugs.gnu.org (full text, mbox):

From: Marius Bakke <marius <at> gnu.org>
To: Alex Griffin <a <at> ajgrf.com>, 41863 <at> debbugs.gnu.org
Cc: "35674 <at> debbugs.gnu.org" <35674 <at> debbugs.gnu.org>,
 "40411 <at> debbugs.gnu.org" <40411 <at> debbugs.gnu.org>
Subject: Re: [bug#41863] [PATCH] services: Fix gdm-autologin pam service.
Date: Fri, 02 Oct 2020 19:54:28 +0200

[Message part 1 (text/plain, inline)]

Alex Griffin via Guix-patches via <guix-patches <at> gnu.org> writes:

> This patch fixes GDM's auto-login feature.
>
> See the discussions in #35674 and #40411. It works for me, but I don't want to just commit it because I don't really understand PAM. (Then again, who does?)

[...]

> @@ -925,7 +926,7 @@ the GNOME desktop environment.")
>      (inherit (unix-pam-service "gdm-autologin"
>                                 #:login-uid? #t))
>      (auth (list (pam-entry
> -                 (control "[success=ok default=1]")
> +                 (control "optional")

This is one of the alternatives Timothy proposed in #35674 back in May
last year(!).  As long as GDM still rejects wrong or blank passwords, I
think it is good to go.

It would be good to get it in 1.2.0.  :-)

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