1===================
2Speculation Control
3===================
4
5Quite some CPUs have speculation-related misfeatures which are in
6fact vulnerabilities causing data leaks in various forms even across
7privilege domains.
8
9The kernel provides mitigation for such vulnerabilities in various
10forms. Some of these mitigations are compile-time configurable and some
11can be supplied on the kernel command line.
12
13There is also a class of mitigations which are very expensive, but they can
14be restricted to a certain set of processes or tasks in controlled
15environments. The mechanism to control these mitigations is via
16:manpage:`prctl(2)`.
17
18There are two prctl options which are related to this:
19
20 * PR_GET_SPECULATION_CTRL
21
22 * PR_SET_SPECULATION_CTRL
23
24PR_GET_SPECULATION_CTRL
25-----------------------
26
27PR_GET_SPECULATION_CTRL returns the state of the speculation misfeature
28which is selected with arg2 of prctl(2). The return value uses bits 0-3 with
29the following meaning:
30
31==== ===================== ===================================================
32Bit  Define                Description
33==== ===================== ===================================================
340    PR_SPEC_PRCTL         Mitigation can be controlled per task by
35                           PR_SET_SPECULATION_CTRL.
361    PR_SPEC_ENABLE        The speculation feature is enabled, mitigation is
37                           disabled.
382    PR_SPEC_DISABLE       The speculation feature is disabled, mitigation is
39                           enabled.
403    PR_SPEC_FORCE_DISABLE Same as PR_SPEC_DISABLE, but cannot be undone. A
41                           subsequent prctl(..., PR_SPEC_ENABLE) will fail.
42==== ===================== ===================================================
43
44If all bits are 0 the CPU is not affected by the speculation misfeature.
45
46If PR_SPEC_PRCTL is set, then the per-task control of the mitigation is
47available. If not set, prctl(PR_SET_SPECULATION_CTRL) for the speculation
48misfeature will fail.
49
50PR_SET_SPECULATION_CTRL
51-----------------------
52
53PR_SET_SPECULATION_CTRL allows to control the speculation misfeature, which
54is selected by arg2 of :manpage:`prctl(2)` per task. arg3 is used to hand
55in the control value, i.e. either PR_SPEC_ENABLE or PR_SPEC_DISABLE or
56PR_SPEC_FORCE_DISABLE.
57
58Common error codes
59------------------
60======= =================================================================
61Value   Meaning
62======= =================================================================
63EINVAL  The prctl is not implemented by the architecture or unused
64        prctl(2) arguments are not 0.
65
66ENODEV  arg2 is selecting a not supported speculation misfeature.
67======= =================================================================
68
69PR_SET_SPECULATION_CTRL error codes
70-----------------------------------
71======= =================================================================
72Value   Meaning
73======= =================================================================
740       Success
75
76ERANGE  arg3 is incorrect, i.e. it's neither PR_SPEC_ENABLE nor
77        PR_SPEC_DISABLE nor PR_SPEC_FORCE_DISABLE.
78
79ENXIO   Control of the selected speculation misfeature is not possible.
80        See PR_GET_SPECULATION_CTRL.
81
82EPERM   Speculation was disabled with PR_SPEC_FORCE_DISABLE and caller
83        tried to enable it again.
84======= =================================================================
85
86Speculation misfeature controls
87-------------------------------
88- PR_SPEC_STORE_BYPASS: Speculative Store Bypass
89
90  Invocations:
91   * prctl(PR_GET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, 0, 0, 0);
92   * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_ENABLE, 0, 0);
93   * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_DISABLE, 0, 0);
94   * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_FORCE_DISABLE, 0, 0);
95