blob: c4dbe6f7cdae8c8c76e706d773322af79cc26b78
1 | =================== |
2 | Speculation Control |
3 | =================== |
4 | |
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. |
8 | |
9 | The kernel provides mitigation for such vulnerabilities in various |
10 | forms. Some of these mitigations are compile-time configurable and some |
11 | can be supplied on 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 0-3 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 | 3 PR_SPEC_FORCE_DISABLE Same as PR_SPEC_DISABLE, but cannot be undone. A |
41 | subsequent prctl(..., PR_SPEC_ENABLE) will fail. |
42 | ==== ===================== =================================================== |
43 | |
44 | If all bits are 0 the CPU is not affected by the speculation misfeature. |
45 | |
46 | If PR_SPEC_PRCTL is set, then the per-task control of the mitigation is |
47 | available. If not set, prctl(PR_SET_SPECULATION_CTRL) for the speculation |
48 | misfeature will fail. |
49 | |
50 | PR_SET_SPECULATION_CTRL |
51 | ----------------------- |
52 | |
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 |
55 | in the control value, i.e. either PR_SPEC_ENABLE or PR_SPEC_DISABLE or |
56 | PR_SPEC_FORCE_DISABLE. |
57 | |
58 | Common error codes |
59 | ------------------ |
60 | ======= ================================================================= |
61 | Value Meaning |
62 | ======= ================================================================= |
63 | EINVAL The prctl is not implemented by the architecture or unused |
64 | prctl(2) arguments are not 0. |
65 | |
66 | ENODEV arg2 is selecting a not supported speculation misfeature. |
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 |
77 | PR_SPEC_DISABLE nor PR_SPEC_FORCE_DISABLE. |
78 | |
79 | ENXIO Control of the selected speculation misfeature is not possible. |
80 | See PR_GET_SPECULATION_CTRL. |
81 | |
82 | EPERM Speculation was disabled with PR_SPEC_FORCE_DISABLE and caller |
83 | tried to enable it again. |
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); |
94 | * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_FORCE_DISABLE, 0, 0); |
95 | |
96 | - PR_SPEC_INDIR_BRANCH: Indirect Branch Speculation in User Processes |
97 | (Mitigate Spectre V2 style attacks against user processes) |
98 | |
99 | Invocations: |
100 | * prctl(PR_GET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, 0, 0, 0); |
101 | * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_ENABLE, 0, 0); |
102 | * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_DISABLE, 0, 0); |
103 | * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_FORCE_DISABLE, 0, 0); |
104 |