Linux kernel before 2.6.18, when running on x86_64 systems, does not properly save or restore EFLAGS during a context switch, which allows local users to cause a denial of service (crash) by causing SYSENTER to set an NT flag, which can trigger a crash on the IRET of the next task.
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A9554
http://www.ubuntu.com/usn/usn-416-1
http://www.securityfocus.com/bid/26060
http://www.securityfocus.com/archive/1/471457
http://www.redhat.com/support/errata/RHSA-2008-0957.html
http://www.mandriva.com/security/advisories?name=MDKSA-2007:171
http://www.kernel.org/pub/linux/kernel/v2.6/snapshots/patch-2.6.18-git13.log
http://www.kernel.org/pub/linux/kernel/v2.6/ChangeLog-2.6.18
http://www.debian.org/security/2007/dsa-1381
http://secunia.com/advisories/32485
http://secunia.com/advisories/26994
http://secunia.com/advisories/26620
http://secunia.com/advisories/25691