| rjw | 1f88458 | 2022-01-06 17:20:42 +0800 | [diff] [blame^] | 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 | .. _set_spec_ctrl: |
| 51 | |
| 52 | PR_SET_SPECULATION_CTRL |
| 53 | ----------------------- |
| 54 | |
| 55 | PR_SET_SPECULATION_CTRL allows to control the speculation misfeature, which |
| 56 | is selected by arg2 of :manpage:`prctl(2)` per task. arg3 is used to hand |
| 57 | in the control value, i.e. either PR_SPEC_ENABLE or PR_SPEC_DISABLE or |
| 58 | PR_SPEC_FORCE_DISABLE. |
| 59 | |
| 60 | Common error codes |
| 61 | ------------------ |
| 62 | ======= ================================================================= |
| 63 | Value Meaning |
| 64 | ======= ================================================================= |
| 65 | EINVAL The prctl is not implemented by the architecture or unused |
| 66 | prctl(2) arguments are not 0. |
| 67 | |
| 68 | ENODEV arg2 is selecting a not supported speculation misfeature. |
| 69 | ======= ================================================================= |
| 70 | |
| 71 | PR_SET_SPECULATION_CTRL error codes |
| 72 | ----------------------------------- |
| 73 | ======= ================================================================= |
| 74 | Value Meaning |
| 75 | ======= ================================================================= |
| 76 | 0 Success |
| 77 | |
| 78 | ERANGE arg3 is incorrect, i.e. it's neither PR_SPEC_ENABLE nor |
| 79 | PR_SPEC_DISABLE nor PR_SPEC_FORCE_DISABLE. |
| 80 | |
| 81 | ENXIO Control of the selected speculation misfeature is not possible. |
| 82 | See PR_GET_SPECULATION_CTRL. |
| 83 | |
| 84 | EPERM Speculation was disabled with PR_SPEC_FORCE_DISABLE and caller |
| 85 | tried to enable it again. |
| 86 | ======= ================================================================= |
| 87 | |
| 88 | Speculation misfeature controls |
| 89 | ------------------------------- |
| 90 | - PR_SPEC_STORE_BYPASS: Speculative Store Bypass |
| 91 | |
| 92 | Invocations: |
| 93 | * prctl(PR_GET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, 0, 0, 0); |
| 94 | * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_ENABLE, 0, 0); |
| 95 | * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_DISABLE, 0, 0); |
| 96 | * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_FORCE_DISABLE, 0, 0); |
| 97 | |
| 98 | - PR_SPEC_INDIR_BRANCH: Indirect Branch Speculation in User Processes |
| 99 | (Mitigate Spectre V2 style attacks against user processes) |
| 100 | |
| 101 | Invocations: |
| 102 | * prctl(PR_GET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, 0, 0, 0); |
| 103 | * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_ENABLE, 0, 0); |
| 104 | * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_DISABLE, 0, 0); |
| 105 | * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_FORCE_DISABLE, 0, 0); |