The exit_thread function (process.c) in Linux kernel 2.6 through 2.6.5 does not invalidate the per-TSS io_bitmap pointers if a process obtains IO access permissions from the ioperm function but does not drop those permissions when it exits, which allows other processes to access the per-TSS pointers, access restricted memory locations, and possibly gain privileges.
https://exchange.xforce.ibmcloud.com/vulnerabilities/16106
http://www.ussg.iu.edu/hypermail/linux/kernel/0405.0/1265.html
http://www.securityfocus.com/bid/10302