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
* Fido: 2:480/124 ** WWW: http://www.frasunek.com/ ** NICHDL: PMF9-RIPE *
* JID: venglin_at_jabber.atman.pl ** PGP ID: 2578FCAD ** HAM-RADIO: SQ8JIV *