The user_update function in security/keys/user_defined.c in the Linux kernel 2.6 allows local users to cause a denial of service (NULL pointer dereference and kernel oops) via vectors related to a user-defined key and "updating a negative key into a fully instantiated key."
https://bugzilla.redhat.com/show_bug.cgi?id=751297
http://www.ubuntu.com/usn/USN-1344-1
http://www.ubuntu.com/usn/USN-1328-1
http://www.ubuntu.com/usn/USN-1324-1
http://www.securityfocus.com/bid/50755
http://www.openwall.com/lists/oss-security/2011/11/22/6
http://www.openwall.com/lists/oss-security/2011/11/22/5
http://www.openwall.com/lists/oss-security/2011/11/21/19