Commit | Line | Data |
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b617cfc8 TG |
1 | =================== |
2 | Speculation Control | |
3 | =================== | |
4 | ||
5 | Quite some CPUs have speculation related misfeatures which are in fact | |
6 | vulnerabilites causing data leaks in various forms even accross privilege | |
7 | domains. | |
8 | ||
9 | The kernel provides mitigation for such vulnerabilities in various | |
10 | forms. Some of these mitigations are compile time configurable and some on | |
11 | the kernel command line. | |
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 | |
28 | which is selected with arg2 of prctl(2). The return value uses bits 0-2 with | |
29 | the following meaning: | |
30 | ||
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 | ==== ================ =================================================== | |
41 | ||
42 | If all bits are 0 the CPU is not affected by the speculation misfeature. | |
43 | ||
44 | If PR_SPEC_PRCTL is set, then the per task control of the mitigation is | |
45 | available. If not set, prctl(PR_SET_SPECULATION_CTRL) for the speculation | |
46 | misfeature will fail. | |
47 | ||
48 | PR_SET_SPECULATION_CTRL | |
49 | ----------------------- | |
50 | PR_SET_SPECULATION_CTRL allows to control the speculation misfeature, which | |
51 | is selected by arg2 of :manpage:`prctl(2)` per task. arg3 is used to hand | |
52 | in the control value, i.e. either PR_SPEC_ENABLE or PR_SPEC_DISABLE. | |
53 | ||
54 | Common error codes | |
55 | ------------------ | |
56 | ======= ================================================================= | |
57 | Value Meaning | |
58 | ======= ================================================================= | |
59 | EINVAL The prctl is not implemented by the architecture or unused | |
60 | prctl(2) arguments are not 0 | |
61 | ||
62 | ENODEV arg2 is selecting a not supported speculation misfeature | |
63 | ======= ================================================================= | |
64 | ||
65 | PR_SET_SPECULATION_CTRL error codes | |
66 | ----------------------------------- | |
67 | ======= ================================================================= | |
68 | Value Meaning | |
69 | ======= ================================================================= | |
70 | 0 Success | |
71 | ||
72 | ERANGE arg3 is incorrect, i.e. it's neither PR_SPEC_ENABLE nor | |
73 | PR_SPEC_DISABLE | |
74 | ||
75 | ENXIO Control of the selected speculation misfeature is not possible. | |
76 | See PR_GET_SPECULATION_CTRL. | |
77 | ======= ================================================================= | |
78 | ||
79 | Speculation misfeature controls | |
80 | ------------------------------- | |
81 | - PR_SPEC_STORE_BYPASS: Speculative Store Bypass | |
82 | ||
83 | Invocations: | |
84 | * prctl(PR_GET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, 0, 0, 0); | |
85 | * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_ENABLE, 0, 0); | |
86 | * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_DISABLE, 0, 0); |