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1 | =================== |
2 | Speculation Control | |
3 | =================== | |
4 | ||
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5 | Quite some CPUs have speculation-related misfeatures which are in |
6 | fact vulnerabilities causing data leaks in various forms even across | |
7 | privilege domains. | |
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8 | |
9 | The kernel provides mitigation for such vulnerabilities in various | |
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10 | forms. Some of these mitigations are compile-time configurable and some |
11 | can be supplied on the kernel command line. | |
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12 | |
13 | There is also a class of mitigations which are very expensive, but they can | |
14 | be restricted to a certain set of processes or tasks in controlled | |
15 | environments. The mechanism to control these mitigations is via | |
16 | :manpage:`prctl(2)`. | |
17 | ||
18 | There are two prctl options which are related to this: | |
19 | ||
20 | * PR_GET_SPECULATION_CTRL | |
21 | ||
22 | * PR_SET_SPECULATION_CTRL | |
23 | ||
24 | PR_GET_SPECULATION_CTRL | |
25 | ----------------------- | |
26 | ||
27 | PR_GET_SPECULATION_CTRL returns the state of the speculation misfeature | |
733f4234 | 28 | which is selected with arg2 of prctl(2). The return value uses bits 0-3 with |
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29 | the following meaning: |
30 | ||
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31 | ==== ===================== =================================================== |
32 | Bit Define Description | |
33 | ==== ===================== =================================================== | |
34 | 0 PR_SPEC_PRCTL Mitigation can be controlled per task by | |
8383394e | 35 | PR_SET_SPECULATION_CTRL. |
733f4234 | 36 | 1 PR_SPEC_ENABLE The speculation feature is enabled, mitigation is |
8383394e | 37 | disabled. |
733f4234 | 38 | 2 PR_SPEC_DISABLE The speculation feature is disabled, mitigation is |
8383394e | 39 | enabled. |
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40 | 3 PR_SPEC_FORCE_DISABLE Same as PR_SPEC_DISABLE, but cannot be undone. A |
41 | subsequent prctl(..., PR_SPEC_ENABLE) will fail. | |
42 | ==== ===================== =================================================== | |
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43 | |
44 | If all bits are 0 the CPU is not affected by the speculation misfeature. | |
45 | ||
8383394e | 46 | If PR_SPEC_PRCTL is set, then the per-task control of the mitigation is |
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47 | available. If not set, prctl(PR_SET_SPECULATION_CTRL) for the speculation |
48 | misfeature will fail. | |
49 | ||
50 | PR_SET_SPECULATION_CTRL | |
51 | ----------------------- | |
733f4234 | 52 | |
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53 | PR_SET_SPECULATION_CTRL allows to control the speculation misfeature, which |
54 | is selected by arg2 of :manpage:`prctl(2)` per task. arg3 is used to hand | |
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55 | in the control value, i.e. either PR_SPEC_ENABLE or PR_SPEC_DISABLE or |
56 | PR_SPEC_FORCE_DISABLE. | |
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57 | |
58 | Common error codes | |
59 | ------------------ | |
60 | ======= ================================================================= | |
61 | Value Meaning | |
62 | ======= ================================================================= | |
63 | EINVAL The prctl is not implemented by the architecture or unused | |
8383394e | 64 | prctl(2) arguments are not 0. |
d34cff3e | 65 | |
8383394e | 66 | ENODEV arg2 is selecting a not supported speculation misfeature. |
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67 | ======= ================================================================= |
68 | ||
69 | PR_SET_SPECULATION_CTRL error codes | |
70 | ----------------------------------- | |
71 | ======= ================================================================= | |
72 | Value Meaning | |
73 | ======= ================================================================= | |
74 | 0 Success | |
75 | ||
76 | ERANGE arg3 is incorrect, i.e. it's neither PR_SPEC_ENABLE nor | |
8383394e | 77 | PR_SPEC_DISABLE nor PR_SPEC_FORCE_DISABLE. |
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78 | |
79 | ENXIO Control of the selected speculation misfeature is not possible. | |
80 | See PR_GET_SPECULATION_CTRL. | |
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81 | |
82 | EPERM Speculation was disabled with PR_SPEC_FORCE_DISABLE and caller | |
83 | tried to enable it again. | |
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84 | ======= ================================================================= |
85 | ||
86 | Speculation 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); | |
733f4234 | 94 | * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_FORCE_DISABLE, 0, 0); |