The __switch_to function in arch/x86/kernel/process_64.c in the Linux kernel through 3.18.1 does not ensure that Thread Local Storage (TLS) descriptors are loaded before proceeding with other steps, which makes it easier for local users to bypass the ASLR protection mechanism via a crafted application that reads a TLS base address.
https://github.com/torvalds/linux/commit/f647d7c155f069c1a068030255c300663516420e
https://bugzilla.redhat.com/show_bug.cgi?id=1177260
http://www.ubuntu.com/usn/USN-2542-1
http://www.ubuntu.com/usn/USN-2541-1
http://www.ubuntu.com/usn/USN-2518-1
http://www.ubuntu.com/usn/USN-2517-1
http://www.ubuntu.com/usn/USN-2516-1
http://www.ubuntu.com/usn/USN-2515-1
http://www.securityfocus.com/bid/71794
http://www.openwall.com/lists/oss-security/2014/12/25/1
http://www.mandriva.com/security/advisories?name=MDVSA-2015:058
http://www.debian.org/security/2015/dsa-3128
http://rhn.redhat.com/errata/RHSA-2015-1081.html
http://lists.opensuse.org/opensuse-security-announce/2015-04/msg00015.html
http://lists.opensuse.org/opensuse-security-announce/2015-04/msg00009.html
http://lists.opensuse.org/opensuse-security-announce/2015-03/msg00020.html
http://lists.fedoraproject.org/pipermail/package-announce/2015-January/147973.html
http://lists.fedoraproject.org/pipermail/package-announce/2015-January/147864.html