Linux kernel 2.6.16-rc2 and earlier, when running on x86_64 systems with preemption enabled, allows local users to cause a denial of service (oops) via multiple ptrace tasks that perform single steps, which can cause corruption of the DEBUG_STACK stack during the do_debug function call.
http://www.securityfocus.com/bid/17216
http://www.mandriva.com/security/advisories?name=MDKSA-2006:151
http://www.debian.org/security/2006/dsa-1017
http://secunia.com/advisories/21614
http://secunia.com/advisories/19955