Problem description and impact
When running on processors supporting Hyper-Threading Technology, it
is possible for a malicious thread to monitor the execution of another
thread.
Information may be disclosed to local users, allowing in many
cases for privilege escalation. For example, on a multi-user
system, it may be possible to steal cryptographic keys used in
applications such as OpenSSH or SSL-enabled web servers.
NOTE: Similar problems may exist in other
simultaneous multithreading implementations, or even some
systems in the absence of simultaneous multithreading.
However, current research has only demonstrated this flaw in
Hyper-Threading Technology, where shared memory caches are
used.
Workaround
Systems not using processors with Hyper-Threading Technology
support are not affected by this issue. On systems which are
affected, the security flaw can be eliminated by setting the
“machdep.hlt_logical_cpus” tunable:

echo “machdep.hlt_logical_cpus=1” >> /boot/loader.conf

The system must be rebooted in order for tunables to take effect.
Use of this workaround is not recommended on “dual-core” systems, as
this workaround will also disable one of the processor
cores.