spec_ctrl.rst 4.23 KB
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===================
Speculation Control
===================

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Quite some CPUs have speculation-related misfeatures which are in
fact vulnerabilities causing data leaks in various forms even across
privilege domains.
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The kernel provides mitigation for such vulnerabilities in various
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forms. Some of these mitigations are compile-time configurable and some
can be supplied on the kernel command line.
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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
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which is selected with arg2 of prctl(2). The return value uses bits 0-3 with
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the following meaning:

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

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

PR_SET_SPECULATION_CTRL
-----------------------
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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
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in the control value, i.e. either PR_SPEC_ENABLE or PR_SPEC_DISABLE or
PR_SPEC_FORCE_DISABLE.
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Common error codes
------------------
======= =================================================================
Value   Meaning
======= =================================================================
EINVAL  The prctl is not implemented by the architecture or unused
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        prctl(2) arguments are not 0.
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ENODEV  arg2 is selecting a not supported speculation misfeature.
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======= =================================================================

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

ERANGE  arg3 is incorrect, i.e. it's neither PR_SPEC_ENABLE nor
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        PR_SPEC_DISABLE nor PR_SPEC_FORCE_DISABLE.
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ENXIO   Control of the selected speculation misfeature is not possible.
        See PR_GET_SPECULATION_CTRL.
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EPERM   Speculation was disabled with PR_SPEC_FORCE_DISABLE and caller
        tried to enable it again.
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======= =================================================================

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);
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   * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_FORCE_DISABLE, 0, 0);
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- 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);