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>

3 files changed, 159 insertions, 32 deletions

diff --git a/dbus/dbus-sysdeps-util-unix.c b/dbus/dbus-sysdeps-util-unix.c
index 0d8a66c7..199eb3dd 100644
--- a/dbus/dbus-sysdeps-util-unix.c
+++ b/dbus/dbus-sysdeps-util-unix.c
@@ -378,53 +378,140 @@ _dbus_change_to_daemon_user  (const char    *user,
 }
 #endif /* !HAVE_LIBAUDIT */
 
+#ifdef HAVE_SETRLIMIT
 
-/**
- * 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
+/* We assume that if we have setrlimit, we also have getrlimit and
+ * struct rlimit.
  */
-void
-_dbus_request_file_descriptor_limit (unsigned int limit)
+
+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)
 {
-#ifdef HAVE_SETRLIMIT
   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
diff --git a/dbus/dbus-sysdeps-util-win.c b/dbus/dbus-sysdeps-util-win.c
index 4678b11e..ec9afbb6 100644
--- a/dbus/dbus-sysdeps-util-win.c
+++ b/dbus/dbus-sysdeps-util-win.c
@@ -258,9 +258,42 @@ _dbus_change_to_daemon_user  (const char    *user,
   return TRUE;
 }
 
+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_request_file_descriptor_limit (unsigned int limit)
+_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
diff --git a/dbus/dbus-sysdeps.h b/dbus/dbus-sysdeps.h
index 47ba2f43..5465128e 100644
--- a/dbus/dbus-sysdeps.h
+++ b/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