An industry-wide issue was found in the way many modern microprocessor handle
speculative access of system registers inaccessible to unprivileged user.
It relies on the presence of a precisely-defined instruction sequence in the
privileged code which allows speculative load of system registers and that such
register value could be subsequently used in speculatively executed instructions that never actually commit (retire).
As a result, an unprivileged attacker could use this flaw to read privileged
system registers by conducting targeted cache side-channel attacks.
This is a hardware issue and is not currently planned to be mitigated in software (in the Linux kernel). As such, we do not plan to provide mitigations for this issue in the kernel packages as shipped with Red Hat Enterprise Linux 5, 6, 7, and Red Hat Enterprise MRG 2.
Future CPU vendor microcode updates may address this issue.