===================
Speculation Control
===================

Quite some CPUs have speculation-related misfeatures which are in
fact vulnerabilities causing data leaks in various forms even across
privilege domains.

The kernel provides mitigation for such vulnerabilities in various
forms. Some of these mitigations are compile-time configurable and some
can be supplied on the kernel command line.

There is also a class of mitigations which are very expensive, but they can
be restricted to a certain set of processes or tasks in controlled
environments. The mechanism to control these mitigations is via
:manpage:`prctl(2)`.

There are two prctl options which are related to this:

 * PR_GET_SPECULATION_CTRL

 * PR_SET_SPECULATION_CTRL

PR_GET_SPECULATION_CTRL
-----------------------

PR_GET_SPECULATION_CTRL returns the state of the speculation misfeature
which is selected with arg2 of prctl(2). The return value uses bits 0-3 with
the following meaning:

==== ===================== ===================================================
Bit  Define                Description
==== ===================== ===================================================
0    PR_SPEC_PRCTL         Mitigation can be controlled per task by
                           PR_SET_SPECULATION_CTRL.
1    PR_SPEC_ENABLE        The speculation feature is enabled, mitigation is
                           disabled.
2    PR_SPEC_DISABLE       The speculation feature is disabled, mitigation is
                           enabled.
3    PR_SPEC_FORCE_DISABLE Same as PR_SPEC_DISABLE, but cannot be undone. A
                           subsequent prctl(..., PR_SPEC_ENABLE) will fail.
==== ===================== ===================================================

If all bits are 0 the CPU is not affected by the speculation misfeature.

If PR_SPEC_PRCTL is set, then the per-task control of the mitigation is
available. If not set, prctl(PR_SET_SPECULATION_CTRL) for the speculation
misfeature will fail.

PR_SET_SPECULATION_CTRL
-----------------------

PR_SET_SPECULATION_CTRL allows to control the speculation misfeature, which
is selected by arg2 of :manpage:`prctl(2)` per task. arg3 is used to hand
in the control value, i.e. either PR_SPEC_ENABLE or PR_SPEC_DISABLE or
PR_SPEC_FORCE_DISABLE.

Common error codes
------------------
======= =================================================================
Value   Meaning
======= =================================================================
EINVAL  The prctl is not implemented by the architecture or unused
        prctl(2) arguments are not 0.

ENODEV  arg2 is selecting a not supported speculation misfeature.
======= =================================================================

PR_SET_SPECULATION_CTRL error codes
-----------------------------------
======= =================================================================
Value   Meaning
======= =================================================================
0       Success

ERANGE  arg3 is incorrect, i.e. it's neither PR_SPEC_ENABLE nor
        PR_SPEC_DISABLE nor PR_SPEC_FORCE_DISABLE.

ENXIO   Control of the selected speculation misfeature is not possible.
        See PR_GET_SPECULATION_CTRL.

EPERM   Speculation was disabled with PR_SPEC_FORCE_DISABLE and caller
        tried to enable it again.
======= =================================================================

Speculation misfeature controls
-------------------------------
- PR_SPEC_STORE_BYPASS: Speculative Store Bypass

  Invocations:
   * prctl(PR_GET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, 0, 0, 0);
   * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_ENABLE, 0, 0);
   * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_DISABLE, 0, 0);
   * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_FORCE_DISABLE, 0, 0);