pam: Add a `pam_malcontent.so` module to enforce time-limited sessions

This involves adding a build-time dependency on PAM.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

.gitlab-ci.yml

@@ -5,7 +5,7 @@ before_script:
                    libxml2-devel dbus-daemon
                    glib2-devel dbus-devel gobject-introspection-devel
                    gettext-devel polkit-devel polkit-gnome git
-                   lcov
+                   lcov pam-devel
   - export LANG=C.UTF-8
 
 stages:

meson.build

@@ -19,12 +19,19 @@ po_dir = join_paths(meson.source_root(), 'po')
 prefix = get_option('prefix')
 bindir = join_paths(prefix, get_option('bindir'))
 datadir = join_paths(prefix, get_option('datadir'))
+libdir = join_paths(prefix, get_option('libdir'))
 libexecdir = join_paths(prefix, get_option('libexecdir'))
 
 # FIXME: This isn’t exposed in accountsservice.pc
 # See https://gitlab.freedesktop.org/accountsservice/accountsservice/merge_requests/16
 accountsserviceinterfacesdir = join_paths(datadir, 'accountsservice', 'interfaces')
 
+# FIXME: pam.pc doesn’t exist
+pamlibdir = get_option('pamlibdir')
+if pamlibdir == ''
+  pamlibdir = join_paths(prefix, libdir.split('/')[-1], 'security')
+endif
+
 dbus = dependency('dbus-1')
 dbusinterfacesdir = dbus.get_pkgconfig_variable('interfaces_dir',
   define_variable: ['datadir', datadir])
@@ -120,4 +127,5 @@ test_env = [
 
 subdir('accounts-service')
 subdir('malcontent-client')
-subdir('libmalcontent')
+subdir('libmalcontent')
+subdir('pam')

meson_options.txt

@@ -3,4 +3,9 @@ option(
   type: 'boolean',
   value: false,
   description: 'enable installed tests'
-)
+)
+option(
+  'pamlibdir',
+  type: 'string',
+  description: 'directory for PAM modules'
+)

pam/meson.build

@@ -0,0 +1,22 @@
+libpam = cc.find_library('pam', required: true)
+libpam_misc = cc.find_library('pam_misc', required: true)
+
+pam_malcontent = shared_library('pam_malcontent',
+  files('pam_malcontent.c'),
+  name_prefix: '',
+  link_args: [
+    '-shared',
+    '-Wl,--version-script=' + join_paths(meson.current_source_dir(), 'pam_malcontent.sym'),
+  ],
+  dependencies: [
+    dependency('gio-2.0', version: '>= 2.44'),
+    dependency('glib-2.0', version: '>= 2.54.2'),
+    dependency('gobject-2.0', version: '>= 2.54'),
+    libmalcontent_dep,
+    libpam,
+    libpam_misc,
+  ],
+  link_depends: files('pam_malcontent.sym'),
+  include_directories: root_inc,
+  install: true,
+  install_dir: pamlibdir)

pam/pam_malcontent.c

@@ -0,0 +1,206 @@
+/* -*- mode: C; c-file-style: "gnu"; indent-tabs-mode: nil; -*-
+ *
+ * Copyright © 2019 Endless Mobile, Inc.
+ *
+ * This library 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.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ *
+ * Authors:
+ *  - Philip Withnall <withnall@endlessm.com>
+ */
+
+#include "config.h"
+
+#define PAM_SM_ACCOUNT
+
+#include <glib.h>
+#include <glib/gi18n-lib.h>
+#include <libmalcontent/malcontent.h>
+#include <pwd.h>
+#include <security/pam_ext.h>
+#include <security/pam_modules.h>
+#include <security/pam_modutil.h>
+#include <syslog.h>
+
+
+/* Example usage:
+ *
+ * Here’s an example of a PAM file which uses `pam_malcontent.so`. Note
+ * that `pam_malcontent.so` must be listed before `pam_systemd.so`, and it must
+ * have type `account`.
+ *
+ * ```
+ * auth     sufficient pam_unix.so nullok try_first_pass
+ * auth     required   pam_deny.so
+ *
+ * account  required   pam_nologin.so
+ * account  sufficient pam_unix.so
+ * account  required   pam_permit.so
+ * -account required pam_malcontent.so
+ *
+ * password sufficient pam_unix.so nullok sha512 shadow try_first_pass try_authtok
+ * password required   pam_deny.so
+ *
+ * -session optional   pam_keyinit.so revoke
+ * -session optional   pam_loginuid.so
+ * -session optional   pam_systemd.so
+ * session  sufficient pam_unix.so
+ * ```
+*/
+
+/* @pw_out is (transfer none) (out) (not optional) */
+static int
+get_user_data (pam_handle_t         *handle,
+               const char          **username_out,
+               const struct passwd **pw_out)
+{
+  const char *username = NULL;
+  struct passwd *pw = NULL;
+  int r;
+
+  g_return_val_if_fail (handle != NULL, PAM_AUTH_ERR);
+  g_return_val_if_fail (username_out != NULL, PAM_AUTH_ERR);
+  g_return_val_if_fail (pw_out != NULL, PAM_AUTH_ERR);
+
+  r = pam_get_user (handle, &username, NULL);
+  if (r != PAM_SUCCESS)
+    {
+      pam_syslog (handle, LOG_ERR, "Failed to get user name.");
+      return r;
+    }
+
+  if (username == NULL || *username == '\0')
+    {
+      pam_syslog (handle, LOG_ERR, "User name not valid.");
+      return PAM_AUTH_ERR;
+    }
+
+  pw = pam_modutil_getpwnam (handle, username);
+  if (pw == NULL)
+    {
+      pam_syslog (handle, LOG_ERR, "Failed to get user data.");
+      return PAM_USER_UNKNOWN;
+    }
+
+  *pw_out = pw;
+  *username_out = username;
+
+  return PAM_SUCCESS;
+}
+
+static void
+runtime_max_sec_free (pam_handle_t *handle,
+                      void         *data,
+                      int           error_status)
+{
+  g_return_if_fail (data != NULL);
+
+  g_free (data);
+}
+
+PAM_EXTERN int
+pam_sm_acct_mgmt (pam_handle_t  *handle,
+                  int            flags,
+                  int            argc,
+                  const char   **argv)
+{
+  int retval;
+  const char *username = NULL;
+  const struct passwd *pw = NULL;
+  g_autoptr(GDBusConnection) connection = NULL;
+  g_autoptr(MctManager) manager = NULL;
+  g_autoptr(MctSessionLimits) limits = NULL;
+  g_autoptr(GError) local_error = NULL;
+  g_autofree gchar *runtime_max_sec_str = NULL;
+  guint64 now = g_get_real_time ();
+  guint64 time_remaining_secs = 0;
+  gboolean time_limit_enabled = FALSE;
+
+  /* Look up the user data from the handle. */
+  retval = get_user_data (handle, &username, &pw);
+  if (retval != PAM_SUCCESS)
+    {
+      /* The error has already been logged. */
+      return retval;
+    }
+
+  if (pw->pw_uid == 0)
+    {
+      /* Always allow root, to avoid a situation where this PAM module prevents
+       * all users logging in with no way of recovery. */
+      pam_info (handle, _("User ‘%s’ has no time limits enabled"), "root");
+      return PAM_SUCCESS;
+    }
+
+  /* Connect to the system bus. */
+  connection = g_bus_get_sync (G_BUS_TYPE_SYSTEM, NULL, &local_error);
+  if (connection == NULL)
+    {
+      pam_error (handle,
+                 _("Error getting session limits for user ‘%s’: %s"),
+                 username, local_error->message);
+      return PAM_SERVICE_ERR;
+    }
+
+  /* Get the time limits on this user’s session usage. */
+  manager = mct_manager_new (connection);
+  limits = mct_manager_get_session_limits (manager, pw->pw_uid,
+                                           MCT_MANAGER_GET_VALUE_FLAGS_NONE,
+                                           NULL, &local_error);
+
+  if (limits == NULL)
+    {
+      if (g_error_matches (local_error, MCT_MANAGER_ERROR,
+                           MCT_MANAGER_ERROR_DISABLED))
+        {
+          return PAM_SUCCESS;
+        }
+      else
+        {
+          pam_error (handle,
+                     _("Error getting session limits for user ‘%s’: %s"),
+                     username, local_error->message);
+          return PAM_SERVICE_ERR;
+        }
+    }
+
+  /* Check if there’s time left. */
+  if (!mct_session_limits_check_time_remaining (limits, now, &time_remaining_secs,
+                                                &time_limit_enabled))
+    {
+      pam_error (handle, _("User ‘%s’ has no time remaining"), username);
+      return PAM_AUTH_ERR;
+    }
+
+  if (!time_limit_enabled)
+    {
+      pam_info (handle, _("User ‘%s’ has no time limits enabled"), username);
+      return PAM_SUCCESS;
+    }
+
+  /* Propagate the remaining time to the `pam_systemd.so` module, which will
+   * end the user’s session when it runs out. */
+  runtime_max_sec_str = g_strdup_printf ("%" G_GUINT64_FORMAT, time_remaining_secs);
+  retval = pam_set_data (handle, "systemd.runtime_max_sec",
+                         g_steal_pointer (&runtime_max_sec_str), runtime_max_sec_free);
+
+  if (retval != PAM_SUCCESS)
+    {
+      pam_error (handle, _("Error setting time limit on login session: %s"),
+                 pam_strerror (handle, retval));
+      return retval;
+    }
+
+  return PAM_SUCCESS;
+}

pam/pam_malcontent.sym

@@ -0,0 +1,26 @@
+/*
+ * Copyright © 2019 Endless Mobile, Inc.
+ *
+ * This library 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.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ *
+ * Authors:
+ *  - Philip Withnall <withnall@endlessm.com>
+ */
+
+{
+global:
+  pam_sm_acct_mgmt;
+local: *;
+};

po/POTFILES.in

@@ -2,3 +2,4 @@
 # Please keep this file sorted alphabetically.
 accounts-service/com.endlessm.ParentalControls.policy
 libmalcontent/manager.c
+pam/pam_malcontent.c