Attachment 105987 Details for Bug 82820 – [PATCH v3] config: reduce DEFAULT_MESSAGE_UNIX_FDS to 16

[patch] [PATCH v3] config: reduce DEFAULT_MESSAGE_UNIX_FDS to 16

0001-config-reduce-DEFAULT_MESSAGE_UNIX_FDS-to-16.patch (text/plain), 3.55 KB, created by Alban Crequy on 2014-09-09 14:53:37 UTC

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Creator: Alban Crequy

Created: 2014-09-09 14:53:37 UTC

Size: 3.55 KB

patch

obsolete

>From 79716ff38a299e7c2628e617c891be3f101e0a9d Mon Sep 17 00:00:00 2001
>From: Alban Crequy <alban.crequy@collabora.co.uk>
>Date: Tue, 19 Aug 2014 15:31:10 +0100
>Subject: [PATCH] config: reduce DEFAULT_MESSAGE_UNIX_FDS to 16
>
>This changes 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.)
>
>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.
>
>https://bugs.freedesktop.org/show_bug.cgi?id=82820
>---
> configure.ac | 8 ++------
> 1 file changed, 2 insertions(+), 6 deletions(-)
>
>diff --git a/configure.ac b/configure.ac
>index cbaf874..1aaa1cb 100644
>--- a/configure.ac
>+++ b/configure.ac
>@@ -1238,12 +1238,8 @@ 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])
>+# Keep the default low to avoid DoS issues, see fd.o #82820
>+DEFAULT_MESSAGE_UNIX_FDS=16
> AC_DEFINE_UNQUOTED([DBUS_DEFAULT_MESSAGE_UNIX_FDS],
>   [$DEFAULT_MESSAGE_UNIX_FDS],
>   [Default for dbus_connection_get_max_message_unix_fds()])
>-- 
>1.8.5.3
>

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