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logind: don't delay login for root even if systemd-user-sessions.service is not activated yet

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If for any reason something goes wrong during the boot process (most likely due
to a network issue), system admins should be allowed to log in to the system to
debug the problem. However due to the login session barrier enforced by
systemd-user-sessions.service for all users, logins for root will be delayed
until a (dbus) timeout expires. Beside being confusing, it's not a nice user
experience to wait for an indefinite period of time (no message is shown) this
and also suggests that something went wrong in the background.

The reason of this delay is due to the fact that all units involved in the
creation of a user session are ordered after systemd-user-sessions.service,
which is subject to network issues. If root needs to log in at that time,
logind is requested to create a new session (via pam_systemd), which ultimately
ends up waiting for systemd-user-session.service to be activated. This has the
bad side effect to block login for root until the dbus call done by pam_systemd
times out and the PAM stack proceeds anyways.

To solve this problem, this patch orders the session scope units and the user
instances only after systemd-user-sessions.service for unprivileged users only.
This commit is contained in:
Franck Bui 2022-07-07 12:02:04 +02:00 committed by Luca Boccassi
parent aa5ae9711e
commit 278e815bfa
7 changed files with 49 additions and 8 deletions
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21
src/login/logind-session.c
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@ -36,6 +36,7 @@
#include "strv.h"
#include "terminal-util.h"
#include "tmpfile-util.h"
#include "uid-alloc-range.h"
#include "user-util.h"
#include "util.h"
@ -645,6 +646,7 @@ static int session_start_scope(Session *s, sd_bus_message *properties, sd_bus_er
assert(s->user);
if (!s->scope) {
_cleanup_strv_free_ char **after = NULL;
_cleanup_free_ char *scope = NULL;
const char *description;
@ -656,6 +658,19 @@ static int session_start_scope(Session *s, sd_bus_message *properties, sd_bus_er
description = strjoina("Session ", s->id, " of User ", s->user->user_record->user_name);
/* We usually want to order session scopes after systemd-user-sessions.service since the
* latter unit is used as login session barrier for unprivileged users. However the barrier
* doesn't apply for root as sysadmin should always be able to log in (and without waiting
* for any timeout to expire) in case something goes wrong during the boot process. Since
* ordering after systemd-user-sessions.service and the user instance is optional we make use
* of STRV_IGNORE with strv_new() to skip these order constraints when needed. */
after = strv_new("systemd-logind.service",
s->user->runtime_dir_service,
!uid_is_system(s->user->user_record->uid) ? "systemd-user-sessions.service" : STRV_IGNORE,
s->class != SESSION_BACKGROUND ? s->user->service : STRV_IGNORE);
if (!after)
return log_oom();
r = manager_start_scope(
s->manager,
scope,
@ -665,11 +680,7 @@ static int session_start_scope(Session *s, sd_bus_message *properties, sd_bus_er
/* These two have StopWhenUnneeded= set, hence add a dep towards them */
STRV_MAKE(s->user->runtime_dir_service,
s->class != SESSION_BACKGROUND ? s->user->service : NULL),
/* And order us after some more */
STRV_MAKE("systemd-logind.service",
"systemd-user-sessions.service",
s->user->runtime_dir_service,
s->class != SESSION_BACKGROUND ? s->user->service : NULL),
after,
user_record_home_directory(s->user->user_record),
properties,
error,

5
units/meson.build
View File

@ -312,6 +312,11 @@ meson.add_install_script('meson-add-wants.sh', add_wants)
install_data('user-.slice.d/10-defaults.conf',
install_dir : systemunitdir + '/user-.slice.d')
install_data('user@.service.d/10-login-barrier.conf',
install_dir : systemunitdir + '/user@.service.d')
install_data('user@0.service.d/10-login-barrier.conf',
install_dir : systemunitdir + '/user@0.service.d')
############################################################
if install_sysconfdir

1
units/user-.slice.d/10-defaults.conf
View File

@ -10,7 +10,6 @@
[Unit]
Description=User Slice of UID %j
Documentation=man:user@.service(5)
After=systemd-user-sessions.service
StopWhenUnneeded=yes
[Slice]

2
units/user-runtime-dir@.service.in
View File

@ -10,7 +10,7 @@
[Unit]
Description=User Runtime Directory /run/user/%i
Documentation=man:user@.service(5)
After=systemd-user-sessions.service dbus.service
After=dbus.service
StopWhenUnneeded=yes
IgnoreOnIsolate=yes

14
units/user@.service.d/10-login-barrier.conf Normal file
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@ -0,0 +1,14 @@
# SPDX-License-Identifier: LGPL-2.1-or-later
#
# This file is part of systemd.
#
# systemd is free software; you can redistribute it and/or modify it
# under the terms of the GNU Lesser General Public License as published by
# the Free Software Foundation; either version 2.1 of the License, or
# (at your option) any later version.
[Unit]
# Make sure user instances are started after logins are allowed. However this
# is not desirable for user@0.service since root should be able to log in
# earlier during the boot process especially if something goes wrong.
After=systemd-user-sessions.service

2
units/user@.service.in
View File

@ -10,7 +10,7 @@
[Unit]
Description=User Manager for UID %i
Documentation=man:user@.service(5)
After=systemd-user-sessions.service user-runtime-dir@%i.service dbus.service
After=user-runtime-dir@%i.service dbus.service
Requires=user-runtime-dir@%i.service
IgnoreOnIsolate=yes

12
units/user@0.service.d/10-login-barrier.conf Normal file
View File

@ -0,0 +1,12 @@
# SPDX-License-Identifier: LGPL-2.1-or-later
#
# This file is part of systemd.
#
# systemd is free software; you can redistribute it and/or modify it
# under the terms of the GNU Lesser General Public License as published by
# the Free Software Foundation; either version 2.1 of the License, or
# (at your option) any later version.
# Empty file to mask its counterpart for unpriviledged users and thus cancels
# "After=systemd-user-session.service" ordering constraint so that root can log
# in even if the boot process is not yet finished.