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6465e37c8f
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> |
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| bus | ||
| dbus | ||
| doc | ||
| modules | ||
| test | ||
| tools | ||
| bus-test.bat.cmake | ||
| CMakeLists.txt | ||
| config.h.cmake | ||
| ConfigureChecks.cmake | ||
| dbus-env.bat.cmake | ||