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prctl: Add force disable speculation

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For certain use cases it is desired to enforce mitigations so they cannot
be undone afterwards. That's important for loader stubs which want to
prevent a child from disabling the mitigation again. Will also be used for
seccomp(). The extra state preserving of the prctl state for SSB is a
preparatory step for EBPF dymanic speculation control.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
This commit is contained in:
Thomas Gleixner 2018-05-03 22:09:15 +02:00
parent f9544b2b07
commit 356e4bfff2
5 changed files with 59 additions and 24 deletions
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34
Documentation/userspace-api/spec_ctrl.rst
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@ -25,19 +25,21 @@ 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 0-2 with
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
==== ================ ===================================================
==== ===================== ===================================================
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.
@ -47,9 +49,11 @@ 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.
in the control value, i.e. either PR_SPEC_ENABLE or PR_SPEC_DISABLE or
PR_SPEC_FORCE_DISABLE.
Common error codes
------------------
@ -70,10 +74,13 @@ Value Meaning
0 Success
ERANGE arg3 is incorrect, i.e. it's neither PR_SPEC_ENABLE nor
PR_SPEC_DISABLE
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
@ -84,3 +91,4 @@ Speculation misfeature controls
* 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);