Oracle Linux: CVE-2018-15572: ELSA-2018-4261: Unbreakable Enterprise kernel security update (IMPORTANT) (Multiple Advisories)

The spectre_v2_select_mitigation function in arch/x86/kernel/cpu/bugs.c in the Linux kernel before 4.18.1 does not always fill RSB upon a context switch, which makes it easier for attackers to conduct userspace-userspace spectreRSB attacks.
An issue was found in the way modern x86 microprocessor designs have implemented speculative execution of instructions (a commonly used performance optimization) and prediction of return addresses via Return Stack Buffer (RSB). RSB is a small on chip structure that holds list of return addresses, used to predict function return addresses. An unprivileged attacker could use this flaw to cross the syscall or process boundary and read privileged memory by conducting targeted cache side-channel attacks.