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.
bugzilla.kernel.org/show_bug.cgi?id=8765
git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git%3Ba=commitdiff%3Bh=29eb51101c02df517ca64ec472d7501127ad1da8
git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git%3Ba=commitdiff%3Bh=a10d9a71bafd3a283da240d2868e71346d2aef6f
osvdb.org/37286
secunia.com/advisories/26935
secunia.com/advisories/26955
secunia.com/advisories/26978
secunia.com/advisories/27322
secunia.com/advisories/29159
wiki.rpath.com/wiki/Advisories:rPSA-2008-0094
www.debian.org/security/2007/dsa-1378
www.redhat.com/support/errata/RHSA-2007-0940.html
www.securityfocus.com/archive/1/488972/100/0/threaded
www.securityfocus.com/bid/25801
www.ubuntu.com/usn/usn-518-1
bugzilla.redhat.com/show_bug.cgi?id=248324
issues.rpath.com/browse/RPL-2304
oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A10394