config: change DEFAULT_MESSAGE_UNIX_FDS to 16

This addresses CVE-2014-3636.

Based on a patch by Alban Crequy. Now that it's the same on all
platforms, there's little point in it being set by configure/cmake.

This change fixes two distinct denials of service:

fd.o#82820, part A
------------------

Before this patch, the system bus had the following default configuration:
- max_connections_per_user: 256
- DBUS_DEFAULT_MESSAGE_UNIX_FDS: usually 1024 (or 256 on QNX, see fd.o#61176)
  as defined by configure.ac
- max_incoming_unix_fds: DBUS_DEFAULT_MESSAGE_UNIX_FDS*4 = usually 4096
- max_outgoing_unix_fds: DBUS_DEFAULT_MESSAGE_UNIX_FDS*4 = usually 4096
- max_message_unix_fds: DBUS_DEFAULT_MESSAGE_UNIX_FDS = usually 1024

This means that a single user could create 256 connections and transmit
256*4096 = 1048576 file descriptors.

The file descriptors stay attached to the dbus-daemon process while they are
in the message loader, in the outgoing queue or waiting to be dispatched before
D-Bus activation.

dbus-daemon is usually limited to 65536 file descriptors (ulimit -n). If the
limit is reached and dbus-daemon needs to receive a message with a file
descriptor attached, this is signalled by recvfrom with the flag MSG_CTRUNC.
Dbus-daemon cannot recover from that error because the kernel does not have any
API to retrieve a file descriptor which has been discarded with MSG_CTRUNC.
Therefore, it closes the connection of the sender. This is not necessarily the
connection which generated the most file descriptors so it can lead to
denial-of-service attacks.

In order to prevent DoS issues, this patch reduces DEFAULT_MESSAGE_UNIX_FDS to
16:

max_connections_per_user * max_incoming_unix_fds = 256 * 64 = 16384

This is less than the usual "ulimit -n" (65536) with a good margin to
accomodate the other sources of file descriptors (stdin/stdout/stderr,
listening sockets, message loader, etc.).

Distributors on non-Linux may need to configure a smaller limit in
system.conf, if their limit on the number of fds is smaller than
Linux's.

fd.o#82820, part B
------------------

On Linux, it's not possible to send more than 253 fds in a single sendmsg()
call: sendmsg() would return -EINVAL.
  #define SCM_MAX_FD      253

SCM_MAX_FD changed value during Linux history:
- it used to be (OPEN_MAX-1)
- commit c09edd6eb (Jul 2007) changed it to 255
- commit bba14de98 (Nov 2010) changed it to 253

Libdbus always sends all of a message's fds, and the beginning
of the message itself, in a single sendmsg() call. Combining these
two, a malicious sender could split a message across two or more
sendmsg() calls to construct a composite message with 254 or more
fds. When dbus-daemon attempted to relay that message to its
recipient in a single sendmsg() call, it would receive EINVAL,
interpret that as a fatal socket error and disconnect the recipient,
resulting in denial of service.

This is fixed by keeping max_message_unix_fds <= SCM_MAX_FD.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=82820
Reviewed-by: Alban Crequy <alban.crequy@collabora.co.uk>

bus/session.conf.in

@@ -49,7 +49,8 @@
   <limit name="max_outgoing_bytes">1000000000</limit>
   <limit name="max_outgoing_unix_fds">250000000</limit>
   <limit name="max_message_size">1000000000</limit>
-  <limit name="max_message_unix_fds">@DEFAULT_MESSAGE_UNIX_FDS@</limit>
+  <!-- We do not override max_message_unix_fds here since the in-kernel
+       limit is also relatively low -->
   <limit name="service_start_timeout">120000</limit>  
   <limit name="auth_timeout">240000</limit>
   <limit name="max_completed_connections">100000</limit>  

cmake/CMakeLists.txt

@@ -417,10 +417,6 @@ endif (WIN32)
 
 set (DBUS_USER )
 
-# In Autotools this has a different default on QNX, but there seems little
-# point in replicating that here; if you're on an unusual Unix, use Autotools.
-set (DEFAULT_MESSAGE_UNIX_FDS 1024)
-
 # This won't work on Windows. It's not meant to - the system bus is
 # meaningless on Windows anyway.
 #

cmake/config.h.cmake

@@ -82,8 +82,6 @@
 # define DBUS_ENABLE_X11_AUTOLAUNCH 1
 #endif
 
-#define DBUS_DEFAULT_MESSAGE_UNIX_FDS @DEFAULT_MESSAGE_UNIX_FDS@
-
 #define _DBUS_VA_COPY_ASSIGN(a1,a2) { a1 = a2; }
 
 #cmakedefine DBUS_VA_COPY_FUNC

configure.ac

@@ -1242,17 +1242,6 @@ if test x$with_valgrind != xno; then
   AC_DEFINE([WITH_VALGRIND], [1], [Define to add Valgrind instrumentation])
 fi
 
-# Determine maximum number of Unix fds which may be passed
-AS_CASE([$host_os],
-  [*qnx*],
-    [DEFAULT_MESSAGE_UNIX_FDS=256],
-  [*],
-    [DEFAULT_MESSAGE_UNIX_FDS=1024])
-AC_DEFINE_UNQUOTED([DBUS_DEFAULT_MESSAGE_UNIX_FDS],
-  [$DEFAULT_MESSAGE_UNIX_FDS],
-  [Default for dbus_connection_get_max_message_unix_fds()])
-AC_SUBST([DEFAULT_MESSAGE_UNIX_FDS])
-
 #### Set up final flags
 LIBDBUS_LIBS="$THREAD_LIBS $NETWORK_libs"
 AC_SUBST([LIBDBUS_LIBS])

dbus/dbus-message.c

@@ -35,6 +35,7 @@
 #include "dbus-list.h"
 #include "dbus-threads-internal.h"
 #ifdef HAVE_UNIX_FD_PASSING
+#include "dbus-sysdeps.h"
 #include "dbus-sysdeps-unix.h"
 #endif
 

dbus/dbus-sysdeps.h

@@ -558,6 +558,14 @@ void _dbus_request_file_descriptor_limit (unsigned int limit);
 const char *
 _dbus_replace_install_prefix (const char *configure_time_path);
 
+/* Do not set this too high: it is a denial-of-service risk.
+ * See <https://bugs.freedesktop.org/show_bug.cgi?id=82820>
+ *
+ * (This needs to be in the non-Unix-specific header so that
+ * the config-parser can use it.)
+ */
+#define DBUS_DEFAULT_MESSAGE_UNIX_FDS 16
+
 /** @} */
 
 DBUS_END_DECLS