The xen_iret function in arch/x86/xen/xen-asm_32.S in the Linux kernel before 3.7.9 on 32-bit Xen paravirt_ops platforms does not properly handle an invalid value in the DS segment register, which allows guest OS users to gain guest OS privileges via a crafted application.
https://github.com/torvalds/linux/commit/13d2b4d11d69a92574a55bfd985cfb0ca77aebdc
https://bugzilla.redhat.com/show_bug.cgi?id=906309
http://www.ubuntu.com/usn/USN-1808-1
http://www.ubuntu.com/usn/USN-1805-1
http://www.ubuntu.com/usn/USN-1797-1
http://www.ubuntu.com/usn/USN-1796-1
http://www.ubuntu.com/usn/USN-1795-1
http://www.openwall.com/lists/oss-security/2013/02/13/10
http://www.mandriva.com/security/advisories?name=MDVSA-2013:176