CVE request: kernel: AF_UNIX: Fix deadlock on connecting to shutdown socket

[Eugene Teo <eugeneteo () kernel sg>]

Quoting from the patch submitted:
"...a deadlock bug in UNIX domain socket, which makes able to DoS
attack against the local machine by non-root users.

...
Why this happens:
 Error checks between unix_socket_connect() and unix_wait_for_peer() are
 inconsistent. The former calls the latter to wait until the backlog is
 processed. Despite the latter returns without doing anything when the
 socket is shutdown, the former doesn't check the shutdown state and
 just retries calling the latter forever."

How to reproduce:
 1. Make a listening AF_UNIX/SOCK_STREAM socket with an abstruct
    namespace(*), and shutdown(2) it.
 2. Repeat connect(2)ing to the listening socket from the other sockets
    until the connection backlog is full-filled.
 3. connect(2) takes the CPU forever. If every core is taken, the
    system hangs.

Reproducer:
http://patchwork.kernel.org/patch/54678/

You will need to add in the missing header files:
#include <string.h>
#include <stdio.h>
#include <sys/un.h>
#include <sys/types.h>
#include <sys/socket.h>

Reference:
https://bugzilla.redhat.com/show_bug.cgi?id=529626

Thanks, Eugene