FreeBSD <= 6.1 kqueue() NULL pointer dereference

FreeBSD <= 6.1 suffers from classical check/use race condition on SMP
systems in kevent() syscall, leading to kernel mode NULL pointer
dereference. It can be triggered by spawning two threads:
1st thread looping on open() and close() syscalls, and the 2nd thread
looping on kevent(), trying to add possibly invalid filedescriptor.

The bug was fixed in 6.1-STABLE, just before release of 6.2-RELEASE, but
was not recognized as security vulnerability.

The following code exploits this vulnerability to run root shell:
http://www.frasunek.com/kqueue.txt

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