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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 | |
356e4bff | 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 | |
35 | PR_SET_SPECULATION_CTRL. | |
36 | 1 PR_SPEC_ENABLE The speculation feature is enabled, mitigation is | |
37 | disabled. | |
38 | 2 PR_SPEC_DISABLE The speculation feature is disabled, mitigation is | |
39 | enabled. | |
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 | 4 PR_SPEC_DISABLE_NOEXEC Same as PR_SPEC_DISABLE, but the state will be | |
43 | cleared on :manpage:`execve(2)`. | |
44 | ==== ====================== ================================================== | |
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45 | |
46 | If all bits are 0 the CPU is not affected by the speculation misfeature. | |
47 | ||
dd079269 | 48 | If PR_SPEC_PRCTL is set, then the per-task control of the mitigation is |
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49 | available. If not set, prctl(PR_SET_SPECULATION_CTRL) for the speculation |
50 | misfeature will fail. | |
51 | ||
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52 | .. _set_spec_ctrl: |
53 | ||
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54 | PR_SET_SPECULATION_CTRL |
55 | ----------------------- | |
356e4bff | 56 | |
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57 | PR_SET_SPECULATION_CTRL allows to control the speculation misfeature, which |
58 | is selected by arg2 of :manpage:`prctl(2)` per task. arg3 is used to hand | |
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59 | in the control value, i.e. either PR_SPEC_ENABLE or PR_SPEC_DISABLE or |
60 | PR_SPEC_FORCE_DISABLE. | |
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61 | |
62 | Common error codes | |
63 | ------------------ | |
64 | ======= ================================================================= | |
65 | Value Meaning | |
66 | ======= ================================================================= | |
67 | EINVAL The prctl is not implemented by the architecture or unused | |
dd079269 | 68 | prctl(2) arguments are not 0. |
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dd079269 | 70 | ENODEV arg2 is selecting a not supported speculation misfeature. |
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71 | ======= ================================================================= |
72 | ||
73 | PR_SET_SPECULATION_CTRL error codes | |
74 | ----------------------------------- | |
75 | ======= ================================================================= | |
76 | Value Meaning | |
77 | ======= ================================================================= | |
78 | 0 Success | |
79 | ||
80 | ERANGE arg3 is incorrect, i.e. it's neither PR_SPEC_ENABLE nor | |
dd079269 | 81 | PR_SPEC_DISABLE nor PR_SPEC_FORCE_DISABLE. |
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82 | |
83 | ENXIO Control of the selected speculation misfeature is not possible. | |
84 | See PR_GET_SPECULATION_CTRL. | |
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85 | |
86 | EPERM Speculation was disabled with PR_SPEC_FORCE_DISABLE and caller | |
87 | tried to enable it again. | |
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88 | ======= ================================================================= |
89 | ||
90 | Speculation misfeature controls | |
91 | ------------------------------- | |
92 | - PR_SPEC_STORE_BYPASS: Speculative Store Bypass | |
93 | ||
94 | Invocations: | |
95 | * prctl(PR_GET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, 0, 0, 0); | |
96 | * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_ENABLE, 0, 0); | |
97 | * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_DISABLE, 0, 0); | |
356e4bff | 98 | * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_FORCE_DISABLE, 0, 0); |
71368af9 | 99 | * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_DISABLE_NOEXEC, 0, 0); |
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100 | |
101 | - PR_SPEC_INDIR_BRANCH: Indirect Branch Speculation in User Processes | |
102 | (Mitigate Spectre V2 style attacks against user processes) | |
103 | ||
104 | Invocations: | |
105 | * prctl(PR_GET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, 0, 0, 0); | |
106 | * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_ENABLE, 0, 0); | |
107 | * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_DISABLE, 0, 0); | |
108 | * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_FORCE_DISABLE, 0, 0); | |
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109 | |
110 | - PR_SPEC_L1D_FLUSH: Flush L1D Cache on context switch out of the task | |
111 | (works only when tasks run on non SMT cores) | |
112 | ||
113 | Invocations: | |
114 | * prctl(PR_GET_SPECULATION_CTRL, PR_SPEC_L1D_FLUSH, 0, 0, 0); | |
115 | * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_L1D_FLUSH, PR_SPEC_ENABLE, 0, 0); | |
116 | * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_L1D_FLUSH, PR_SPEC_DISABLE, 0, 0); |