The Linux kernel 2.6.20 and 2.6.21 does not properly handle an invalid LDT segment selector in %cs (the xcs field) during ptrace single-step operations, which allows local users to cause a denial of service (NULL dereference and OOPS) via certain code that makes ptrace PTRACE_SETREGS and PTRACE_SINGLESTEP requests, related to the TRACE_IRQS_ON function, and possibly related to the arch_ptrace function.
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A10394
https://issues.rpath.com/browse/RPL-2304
https://bugzilla.redhat.com/show_bug.cgi?id=248324
http://www.ubuntu.com/usn/usn-518-1
http://www.securityfocus.com/bid/25801
http://www.securityfocus.com/archive/1/488972/100/0/threaded
http://www.redhat.com/support/errata/RHSA-2007-0940.html
http://www.debian.org/security/2007/dsa-1378
http://wiki.rpath.com/wiki/Advisories:rPSA-2008-0094
http://secunia.com/advisories/29159
http://secunia.com/advisories/27322
http://secunia.com/advisories/26978
http://secunia.com/advisories/26955