109 lines
4.5 KiB
ReStructuredText
109 lines
4.5 KiB
ReStructuredText
|
===================
|
||
|
|
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.
|
||
|
|
4 PR_SPEC_DISABLE_NOEXEC Same as PR_SPEC_DISABLE, but the state will be
|
||
|
|
cleared on :manpage:`execve(2)`.
|
||
|
|
==== ====================== ==================================================
|
||
|
|
|
||
|
|
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.
|
||
|
|
|
||
|
|
.. _set_spec_ctrl:
|
||
|
|
|
||
|
|
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);
|
||
|
|
* prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_DISABLE_NOEXEC, 0, 0);
|
||
|
|
|
||
|
|
- PR_SPEC_INDIR_BRANCH: Indirect Branch Speculation in User Processes
|
||
|
|
(Mitigate Spectre V2 style attacks against user processes)
|
||
|
|
|
||
|
|
Invocations:
|
||
|
|
* prctl(PR_GET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, 0, 0, 0);
|
||
|
|
* prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_ENABLE, 0, 0);
|
||
|
|
* prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_DISABLE, 0, 0);
|
||
|
|
* prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_FORCE_DISABLE, 0, 0);
|