logind: enforce a limit on current user sessions

We really should put limits on all resources we manage, hence add one to the
number of concurrent sessions, too. This was previously unbounded, hence set a
relatively high limit of 8K by default.

Note that most PAM setups will actually invoke pam_systemd prefixed with "-",
so that the return code of pam_systemd is ignored, and the login attempt
succeeds anyway. On systems like this the session will be created but is not
tracked by systemd.

1 files changed, 23 insertions, 0 deletions

diff --git a/src/login/logind-dbus.c b/src/login/logind-dbus.c
index a281f99a34..5067e5d1a8 100644
--- a/src/login/logind-dbus.c
+++ b/src/login/logind-dbus.c
@@ -265,6 +265,24 @@ static int property_get_docked(
         return sd_bus_message_append(reply, "b", manager_is_docked_or_external_displays(m));
 }
 
+static int property_get_current_sessions(
+                sd_bus *bus,
+                const char *path,
+                const char *interface,
+                const char *property,
+                sd_bus_message *reply,
+                void *userdata,
+                sd_bus_error *error) {
+
+        Manager *m = userdata;
+
+        assert(bus);
+        assert(reply);
+        assert(m);
+
+        return sd_bus_message_append(reply, "t", (uint64_t) hashmap_size(m->sessions));
+}
+
 static int method_get_session(sd_bus_message *message, void *userdata, sd_bus_error *error) {
         _cleanup_free_ char *p = NULL;
         Manager *m = userdata;
@@ -725,6 +743,9 @@ static int method_create_session(sd_bus_message *message, void *userdata, sd_bus
             m->seat0->positions[vtnr]->class != SESSION_GREETER)
                 return sd_bus_error_setf(error, BUS_ERROR_SESSION_BUSY, "Already occupied by a session");
 
+        if (hashmap_size(m->sessions) >= m->sessions_max)
+                return sd_bus_error_setf(error, SD_BUS_ERROR_LIMITS_EXCEEDED, "Maximum number of sessions (%" PRIu64 ") reached, refusing further sessions.", m->sessions_max);
+
         audit_session_from_pid(leader, &audit_id);
         if (audit_id > 0) {
                 /* Keep our session IDs and the audit session IDs in sync */
@@ -2512,6 +2533,8 @@ const sd_bus_vtable manager_vtable[] = {
         SD_BUS_PROPERTY("PreparingForSleep", "b", property_get_preparing, 0, 0),
         SD_BUS_PROPERTY("ScheduledShutdown", "(st)", property_get_scheduled_shutdown, 0, 0),
         SD_BUS_PROPERTY("Docked", "b", property_get_docked, 0, 0),
+        SD_BUS_PROPERTY("SessionsMax", "t", NULL, offsetof(Manager, sessions_max), SD_BUS_VTABLE_PROPERTY_CONST),
+        SD_BUS_PROPERTY("NCurrentSessions", "t", property_get_current_sessions, 0, 0),
 
         SD_BUS_METHOD("GetSession", "s", "o", method_get_session, SD_BUS_VTABLE_UNPRIVILEGED),
         SD_BUS_METHOD("GetSessionByPID", "u", "o", method_get_session_by_pid, SD_BUS_VTABLE_UNPRIVILEGED),