daemon: Define local users as being exactly those present in /etc/shadow

According to https://bugs.freedesktop.org/show_bug.cgi?id=48177 and
https://gitlab.freedesktop.org/accountsservice/accountsservice/-/merge_requests/116,
the intention is that merely existing in /etc/passwd is not enough to
consider an account to be local; it must also be listed in /etc/shadow.
This was done to provide graceful handling of systems where the
complete list of LDAP/NIS/etc. users is written into /etc/passwd by
rsync or similar instead of using a NSS plugin (but authentication still
uses a PAM plugin). However, this unintentionally regressed in 34bedecf
which continues reading after an account not in /etc/shadow is found.

entry_generator_fgetpwent() intentionally only outputs a maximum of 50
users, and only outputs users that are classified as likely to be human
users' accounts, as opposed to system uids. However, when enumerating
cached or explicitly requested users, we need to look them up in a
complete list of local users. Otherwise, we can incorrectly classify
local users as remote (if they are beyond the limit of 50 or have a
username or shell that is more typically used for system users), which
makes at least GNOME Settings display a misleading user interface for
those users.

Resolves: https://gitlab.freedesktop.org/accountsservice/accountsservice/-/issues/107
Bug-Debian: https://bugs.debian.org/1030262
Signed-off-by: Simon McVittie <smcv@debian.org>

1 files changed, 27 insertions, 18 deletions

diff --git a/src/daemon.c b/src/daemon.c
index 0150844..81bf5ad 100644
--- a/src/daemon.c
+++ b/src/daemon.c
@@ -90,7 +90,8 @@ typedef struct
 typedef struct passwd * (* EntryGeneratorFunc) (Daemon *,
                                                 GHashTable *,
                                                 gpointer *,
-                                                struct spwd **shadow_entry);
+                                                struct spwd **shadow_entry,
+                                                GHashTable  **local_users);
 
 typedef struct
 {
@@ -177,7 +178,8 @@ static struct passwd *
 entry_generator_fgetpwent (Daemon       *daemon,
                            GHashTable   *users,
                            gpointer     *state,
-                           struct spwd **spent)
+                           struct spwd **spent,
+                           GHashTable  **local_users)
 {
         struct passwd *pwent;
 
@@ -191,11 +193,14 @@ entry_generator_fgetpwent (Daemon       *daemon,
         {
                 FILE       *fp;
                 /* Local user accounts (currently defined as existing in
-                 * /etc/shadow)
+                 * /etc/shadow, so sites that rsync NIS/LDAP users into
+                 * /etc/passwd don't get them all treated as local)
                  * username -> copy of shadow_entry_buffers */
                 GHashTable *local_users;
         } *generator_state;
 
+        g_assert (local_users == NULL || *local_users == NULL);
+
         /* First iteration */
         if (*state == NULL) {
                 GHashTable *shadow_users = NULL;
@@ -268,7 +273,7 @@ entry_generator_fgetpwent (Daemon       *daemon,
                         if (!user_classify_is_human (pwent->pw_uid, pwent->pw_name, pwent->pw_shell)) {
                                 g_debug ("skipping user: %s", pwent->pw_name);
 
-                                return entry_generator_fgetpwent (daemon, users, state, spent);
+                                return entry_generator_fgetpwent (daemon, users, state, spent, local_users);
                         }
 
                         return pwent;
@@ -276,6 +281,10 @@ entry_generator_fgetpwent (Daemon       *daemon,
         }
 
         /* Last iteration */
+        if (local_users != NULL) {
+                *local_users = g_hash_table_ref (generator_state->local_users);
+        }
+
         fclose (generator_state->fp);
         g_hash_table_unref (generator_state->local_users);
         g_free (generator_state);
@@ -288,7 +297,8 @@ static struct passwd *
 entry_generator_cachedir (Daemon       *daemon,
                           GHashTable   *users,
                           gpointer     *state,
-                          struct spwd **shadow_entry)
+                          struct spwd **shadow_entry,
+                          GHashTable  **local_users)
 {
         struct passwd *pwent;
 
@@ -363,7 +373,8 @@ static struct passwd *
 entry_generator_requested_users (Daemon       *daemon,
                                  GHashTable   *users,
                                  gpointer     *state,
-                                 struct spwd **shadow_entry)
+                                 struct spwd **shadow_entry,
+                                 GHashTable  **local_users)
 {
         DaemonPrivate *priv = daemon_get_instance_private (daemon);
         struct passwd *pwent;
@@ -409,7 +420,8 @@ static void
 load_entries (Daemon            *daemon,
               GHashTable        *users,
               gboolean           explicitly_requested,
-              EntryGeneratorFunc entry_generator)
+              EntryGeneratorFunc entry_generator,
+              GHashTable       **local_users)
 {
         DaemonPrivate *priv = daemon_get_instance_private (daemon);
         gpointer generator_state = NULL;
@@ -418,10 +430,11 @@ load_entries (Daemon            *daemon,
         User *user = NULL;
 
         g_assert (entry_generator != NULL);
+        g_assert (local_users == NULL || *local_users == NULL);
 
         for (;;) {
                 spent = NULL;
-                pwent = entry_generator (daemon, users, &generator_state, &spent);
+                pwent = entry_generator (daemon, users, &generator_state, &spent, local_users);
                 if (pwent == NULL)
                         break;
 
@@ -474,7 +487,7 @@ reload_users (Daemon *daemon)
         gboolean had_no_users, has_no_users, had_multiple_users, has_multiple_users;
         GHashTable *users;
         GHashTable *old_users;
-        GHashTable *local;
+        GHashTable *local = NULL;
         GHashTableIter iter;
         gsize number_of_normal_users = 0;
         gpointer name, value;
@@ -488,19 +501,15 @@ reload_users (Daemon *daemon)
          * them below.
          */
 
-        /* Load the local users into our hash table */
-        load_entries (daemon, users, FALSE, entry_generator_fgetpwent);
-        local = g_hash_table_new (g_str_hash, g_str_equal);
-        g_hash_table_iter_init (&iter, users);
-        while (g_hash_table_iter_next (&iter, &name, NULL)) {
-                g_hash_table_add (local, name);
-        }
+        /* Load the local users into our hash tables */
+        load_entries (daemon, users, FALSE, entry_generator_fgetpwent, &local);
+        g_assert (local != NULL);
 
         /* Now add/update users from other sources, possibly non-local */
-        load_entries (daemon, users, TRUE, entry_generator_cachedir);
+        load_entries (daemon, users, TRUE, entry_generator_cachedir, NULL);
 
         /* and add users to hash table that were explicitly requested  */
-        load_entries (daemon, users, TRUE, entry_generator_requested_users);
+        load_entries (daemon, users, TRUE, entry_generator_requested_users, NULL);
 
         wtmp_helper_update_login_frequencies (users);