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4992ed7813
If you force-disable mitigations on the kernel cmdline, for SPEC_STORE_BYPASS this ends up with the prctl returning -ENXIO, but contrary to the current docs for the other controls it returns -EPERM. Fix that. Note that this return value should probably be considered a bug. But, making the behaviour consistent with the current docs seems more likely to break existing users than help anyone out in practice, so just "fix" it by specifying it as correct. Since this is getting more wordy and confusing, also be more explicit about "control is not possible" be mentioning the boot configuration, to better distinguish this case conceptually from the FORCE_DISABLE failure mode. Signed-off-by: Brendan Jackman <jackmanb@google.com> Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de> Link: https://patch.msgid.link/20251111-b4-prctl-docs-2-v2-1-bc9d14ec9662@google.com
125 lines
5.3 KiB
ReStructuredText
125 lines
5.3 KiB
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===================
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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 (with the caveat that PR_SPEC_L1D_FLUSH has less obvious
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semantics, see documentation for that specific control below):
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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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.. _set_spec_ctrl:
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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 For PR_SPEC_STORE_BYPASS: control of the selected speculation misfeature
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is not possible via prctl, because of the system's boot configuration.
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EPERM Speculation was disabled with PR_SPEC_FORCE_DISABLE and caller tried to
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enable it again.
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EPERM For PR_SPEC_L1D_FLUSH and PR_SPEC_INDIRECT_BRANCH: control of the
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mitigation is not possible because of the system's boot configuration.
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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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- PR_SPEC_L1D_FLUSH: Flush L1D Cache on context switch out of the task
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(works only when tasks run on non SMT cores)
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For this control, PR_SPEC_ENABLE means that the **mitigation** is enabled (L1D
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is flushed), PR_SPEC_DISABLE means it is disabled.
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Invocations:
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* prctl(PR_GET_SPECULATION_CTRL, PR_SPEC_L1D_FLUSH, 0, 0, 0);
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* prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_L1D_FLUSH, PR_SPEC_ENABLE, 0, 0);
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* prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_L1D_FLUSH, PR_SPEC_DISABLE, 0, 0);
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