107 lines
4.5 KiB
ReStructuredText
107 lines
4.5 KiB
ReStructuredText
===================
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Speculation Control
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===================
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Quite some CPUs have speculation-related misfeatures which are in
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fact vulnerabilities causing data leaks in various forms even across
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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
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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
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be restricted to a certain set of processes or tasks in controlled
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environments. The mechanism to control these mitigations is via
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:manpage:`prctl(2)`.
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There are two prctl options which are related to this:
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* PR_GET_SPECULATION_CTRL
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* PR_SET_SPECULATION_CTRL
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PR_GET_SPECULATION_CTRL
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-----------------------
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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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==== ====================== ==================================================
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Bit Define Description
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==== ====================== ==================================================
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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
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subsequent prctl(..., PR_SPEC_ENABLE) will fail.
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4 PR_SPEC_DISABLE_NOEXEC Same as PR_SPEC_DISABLE, but the state will be
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cleared on :manpage:`execve(2)`.
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==== ====================== ==================================================
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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
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misfeature will fail.
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PR_SET_SPECULATION_CTRL
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-----------------------
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PR_SET_SPECULATION_CTRL allows to control the speculation misfeature, which
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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
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PR_SPEC_FORCE_DISABLE.
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Common error codes
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------------------
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======= =================================================================
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Value Meaning
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======= =================================================================
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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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======= =================================================================
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PR_SET_SPECULATION_CTRL error codes
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-----------------------------------
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======= =================================================================
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Value Meaning
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======= =================================================================
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0 Success
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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.
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See PR_GET_SPECULATION_CTRL.
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EPERM Speculation was disabled with PR_SPEC_FORCE_DISABLE and caller
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tried to enable it again.
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======= =================================================================
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Speculation misfeature controls
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-------------------------------
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- PR_SPEC_STORE_BYPASS: Speculative Store Bypass
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Invocations:
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* prctl(PR_GET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, 0, 0, 0);
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* prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_ENABLE, 0, 0);
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* 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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* prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_DISABLE_NOEXEC, 0, 0);
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- PR_SPEC_INDIR_BRANCH: Indirect Branch Speculation in User Processes
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(Mitigate Spectre V2 style attacks against user processes)
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Invocations:
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* prctl(PR_GET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, 0, 0, 0);
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* prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_ENABLE, 0, 0);
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* prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_DISABLE, 0, 0);
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* prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_FORCE_DISABLE, 0, 0);
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