CVE-2014-7824: set fd rlimit to 64k for the system dbus-daemon

This ensures that our rlimit is actually high enough to avoid the
denial of service described in CVE-2014-3636 part A.
CVE-2014-7824 has been allocated for this incomplete fix.

Restore the original rlimit for activated services, to avoid
them getting undesired higher limits.

(Thanks to Alban Crequy for various adjustments which have been
included in this commit.)

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=85105
Reviewed-by: Alban Crequy <alban.crequy@collabora.co.uk>

bus/activation.c

@@ -1688,6 +1688,31 @@ out:
   return retval;
 }
 
+static void
+child_setup (void *user_data)
+{
+#ifdef DBUS_UNIX
+  BusActivation *activation = user_data;
+  DBusRLimit *initial_fd_limit;
+  DBusError error;
+
+  dbus_error_init (&error);
+  initial_fd_limit = bus_context_get_initial_fd_limit (activation->context);
+
+  if (initial_fd_limit != NULL &&
+      !_dbus_rlimit_restore_fd_limit (initial_fd_limit, &error))
+    {
+      /* unfortunately we don't actually know the service name here */
+      bus_context_log (activation->context,
+                       DBUS_SYSTEM_LOG_INFO,
+                       "Failed to reset fd limit before activating "
+                       "service: %s: %s",
+                       error.name, error.message);
+    }
+#endif
+}
+
+
 dbus_bool_t
 bus_activation_activate_service (BusActivation  *activation,
                                  DBusConnection *connection,
@@ -2121,7 +2146,8 @@ bus_activation_activate_service (BusActivation  *activation,
                                           service_name,
                                           argv,
                                           envp,
-                                          NULL, activation,
+                                          child_setup,
+                                          activation,
                                           &tmp_error))
     {
       _dbus_verbose ("Failed to spawn child\n");

bus/bus.c

@@ -64,6 +64,7 @@ struct BusContext
   BusPolicy *policy;
   BusMatchmaker *matchmaker;
   BusLimits limits;
+  DBusRLimit *initial_fd_limit;
   unsigned int fork : 1;
   unsigned int syslog : 1;
   unsigned int keep_umask : 1;
@@ -659,19 +660,38 @@ oom:
 static void
 raise_file_descriptor_limit (BusContext      *context)
 {
+#ifdef DBUS_UNIX
+  DBusError error = DBUS_ERROR_INIT;
 
-  /* I just picked this out of thin air; we need some extra
-   * descriptors for things like any internal pipes we create,
-   * inotify, connections to SELinux, etc.
+  /* we only do this once */
+  if (context->initial_fd_limit != NULL)
+    return;
+
+  context->initial_fd_limit = _dbus_rlimit_save_fd_limit (&error);
+
+  if (context->initial_fd_limit == NULL)
+    {
+      bus_context_log (context, DBUS_SYSTEM_LOG_INFO,
+                       "%s: %s", error.name, error.message);
+      dbus_error_free (&error);
+      return;
+    }
+
+  /* We used to compute a suitable rlimit based on the configured number
+   * of connections, but that breaks down as soon as we allow fd-passing,
+   * because each connection is allowed to pass 64 fds to us, and if
+   * they all did, we'd hit kernel limits. We now hard-code 64k as a
+   * good limit, like systemd does: that's enough to avoid DoS from
+   * anything short of multiple uids conspiring against us.
    */
-  unsigned int arbitrary_extra_fds = 32;
-  unsigned int limit;
-
-  limit = context->limits.max_completed_connections +
-    context->limits.max_incomplete_connections
-    + arbitrary_extra_fds;
-
-  _dbus_request_file_descriptor_limit (limit);
+  if (!_dbus_rlimit_raise_fd_limit_if_privileged (65536, &error))
+    {
+      bus_context_log (context, DBUS_SYSTEM_LOG_INFO,
+                       "%s: %s", error.name, error.message);
+      dbus_error_free (&error);
+      return;
+    }
+#endif
 }
 
 static dbus_bool_t
@@ -1130,6 +1150,10 @@ bus_context_unref (BusContext *context)
 
           dbus_free (context->pidfile);
 	}
+
+      if (context->initial_fd_limit)
+        _dbus_rlimit_free (context->initial_fd_limit);
+
       dbus_free (context);
 
       dbus_server_free_data_slot (&server_data_slot);
@@ -1294,6 +1318,12 @@ bus_context_get_reply_timeout (BusContext *context)
   return context->limits.reply_timeout;
 }
 
+DBusRLimit *
+bus_context_get_initial_fd_limit (BusContext *context)
+{
+  return context->initial_fd_limit;
+}
+
 void
 bus_context_log (BusContext *context, DBusSystemLogSeverity severity, const char *msg, ...) _DBUS_GNUC_PRINTF (3, 4);
 

bus/bus.h

@@ -116,6 +116,7 @@ int               bus_context_get_max_services_per_connection    (BusContext
 int               bus_context_get_max_match_rules_per_connection (BusContext       *context);
 int               bus_context_get_max_replies_per_connection     (BusContext       *context);
 int               bus_context_get_reply_timeout                  (BusContext       *context);
+DBusRLimit *      bus_context_get_initial_fd_limit               (BusContext       *context);
 void              bus_context_log                                (BusContext       *context,
                                                                   DBusSystemLogSeverity severity,
                                                                   const char       *msg,

dbus/dbus-sysdeps-util-unix.c

@@ -378,53 +378,140 @@ _dbus_change_to_daemon_user  (const char    *user,
 }
 #endif /* !HAVE_LIBAUDIT */
 
-
-/**
- * Attempt to ensure that the current process can open
- * at least @p limit file descriptors.
- *
- * If @p limit is lower than the current, it will not be
- * lowered.  No error is returned if the request can
- * not be satisfied.
- *
- * @param limit number of file descriptors
- */
-void
-_dbus_request_file_descriptor_limit (unsigned int limit)
-{
 #ifdef HAVE_SETRLIMIT
+
+/* We assume that if we have setrlimit, we also have getrlimit and
+ * struct rlimit.
+ */
+
+struct DBusRLimit {
+    struct rlimit lim;
+};
+
+DBusRLimit *
+_dbus_rlimit_save_fd_limit (DBusError *error)
+{
+  DBusRLimit *self;
+
+  self = dbus_new0 (DBusRLimit, 1);
+
+  if (self == NULL)
+    {
+      _DBUS_SET_OOM (error);
+      return NULL;
+    }
+
+  if (getrlimit (RLIMIT_NOFILE, &self->lim) < 0)
+    {
+      dbus_set_error (error, _dbus_error_from_errno (errno),
+                      "Failed to get fd limit: %s", _dbus_strerror (errno));
+      dbus_free (self);
+      return NULL;
+    }
+
+  return self;
+}
+
+dbus_bool_t
+_dbus_rlimit_raise_fd_limit_if_privileged (unsigned int  desired,
+                                           DBusError    *error)
+{
   struct rlimit lim;
-  struct rlimit target_lim;
 
   /* No point to doing this practically speaking
    * if we're not uid 0.  We expect the system
    * bus to use this before we change UID, and
-   * the session bus takes the Linux default
-   * of 1024 for both cur and max.
+   * the session bus takes the Linux default,
+   * currently 1024 for cur and 4096 for max.
    */
   if (getuid () != 0)
-    return;
+    {
+      /* not an error, we're probably the session bus */
+      return TRUE;
+    }
 
   if (getrlimit (RLIMIT_NOFILE, &lim) < 0)
-    return;
+    {
+      dbus_set_error (error, _dbus_error_from_errno (errno),
+                      "Failed to get fd limit: %s", _dbus_strerror (errno));
+      return FALSE;
+    }
 
-  if (lim.rlim_cur >= limit)
-    return;
+  if (lim.rlim_cur == RLIM_INFINITY || lim.rlim_cur >= desired)
+    {
+      /* not an error, everything is fine */
+      return TRUE;
+    }
 
   /* Ignore "maximum limit", assume we have the "superuser"
    * privileges.  On Linux this is CAP_SYS_RESOURCE.
    */
-  target_lim.rlim_cur = target_lim.rlim_max = limit;
-  /* Also ignore errors; if we fail, we will at least work
-   * up to whatever limit we had, which seems better than
-   * just outright aborting.
-   *
-   * However, in the future we should probably log this so OS builders
-   * have a chance to notice any misconfiguration like dbus-daemon
-   * being started without CAP_SYS_RESOURCE.
-   */
-  setrlimit (RLIMIT_NOFILE, &target_lim);
+  lim.rlim_cur = lim.rlim_max = desired;
+
+  if (setrlimit (RLIMIT_NOFILE, &lim) < 0)
+    {
+      dbus_set_error (error, _dbus_error_from_errno (errno),
+                      "Failed to set fd limit to %u: %s",
+                      desired, _dbus_strerror (errno));
+      return FALSE;
+    }
+
+  return TRUE;
+}
+
+dbus_bool_t
+_dbus_rlimit_restore_fd_limit (DBusRLimit *saved,
+                               DBusError  *error)
+{
+  if (setrlimit (RLIMIT_NOFILE, &saved->lim) < 0)
+    {
+      dbus_set_error (error, _dbus_error_from_errno (errno),
+                      "Failed to restore old fd limit: %s",
+                      _dbus_strerror (errno));
+      return FALSE;
+    }
+
+  return TRUE;
+}
+
+#else /* !HAVE_SETRLIMIT */
+
+static void
+fd_limit_not_supported (DBusError *error)
+{
+  dbus_set_error (error, DBUS_ERROR_NOT_SUPPORTED,
+                  "cannot change fd limit on this platform");
+}
+
+DBusRLimit *
+_dbus_rlimit_save_fd_limit (DBusError *error)
+{
+  fd_limit_not_supported (error);
+  return NULL;
+}
+
+dbus_bool_t
+_dbus_rlimit_raise_fd_limit_if_privileged (unsigned int  desired,
+                                           DBusError    *error)
+{
+  fd_limit_not_supported (error);
+  return FALSE;
+}
+
+dbus_bool_t
+_dbus_rlimit_restore_fd_limit (DBusRLimit *saved,
+                               DBusError  *error)
+{
+  fd_limit_not_supported (error);
+  return FALSE;
+}
+
 #endif
+
+void
+_dbus_rlimit_free (DBusRLimit *lim)
+{
+  dbus_free (lim);
 }
 
 void

dbus/dbus-sysdeps-util-win.c

@@ -258,9 +258,42 @@ _dbus_change_to_daemon_user  (const char    *user,
   return TRUE;
 }
 
-void
-_dbus_request_file_descriptor_limit (unsigned int limit)
+static void
+fd_limit_not_supported (DBusError *error)
 {
+  dbus_set_error (error, DBUS_ERROR_NOT_SUPPORTED,
+                  "cannot change fd limit on this platform");
+}
+
+DBusRLimit *
+_dbus_rlimit_save_fd_limit (DBusError *error)
+{
+  fd_limit_not_supported (error);
+  return NULL;
+}
+
+dbus_bool_t
+_dbus_rlimit_raise_fd_limit_if_privileged (unsigned int  desired,
+                                           DBusError    *error)
+{
+  fd_limit_not_supported (error);
+  return FALSE;
+}
+
+dbus_bool_t
+_dbus_rlimit_restore_fd_limit (DBusRLimit *saved,
+                               DBusError  *error)
+{
+  fd_limit_not_supported (error);
+  return FALSE;
+}
+
+void
+_dbus_rlimit_free (DBusRLimit *lim)
+{
+  /* _dbus_rlimit_save_fd_limit() cannot return non-NULL on Windows
+   * so there cannot be anything to free */
+  _dbus_assert (lim == NULL);
 }
 
 void

dbus/dbus-sysdeps.h

@@ -546,8 +546,6 @@ dbus_bool_t _dbus_change_to_daemon_user (const char *user,
 
 void _dbus_flush_caches (void);
 
-void _dbus_request_file_descriptor_limit (unsigned int limit);
-
 /*
  * replaces the term DBUS_PREFIX in configure_time_path by the
  * current dbus installation directory. On unix this function is a noop
@@ -566,6 +564,15 @@ _dbus_replace_install_prefix (const char *configure_time_path);
  */
 #define DBUS_DEFAULT_MESSAGE_UNIX_FDS 16
 
+typedef struct DBusRLimit DBusRLimit;
+
+DBusRLimit     *_dbus_rlimit_save_fd_limit                 (DBusError    *error);
+dbus_bool_t     _dbus_rlimit_raise_fd_limit_if_privileged  (unsigned int  desired,
+                                                            DBusError    *error);
+dbus_bool_t     _dbus_rlimit_restore_fd_limit              (DBusRLimit   *saved,
+                                                            DBusError    *error);
+void            _dbus_rlimit_free                          (DBusRLimit   *lim);
+
 /** @} */
 
 DBUS_END_DECLS