1===================
  2Speculation Control
  3===================
  4
  5Quite some CPUs have speculation-related misfeatures which are in
  6fact vulnerabilities causing data leaks in various forms even across
  7privilege domains.
  8
  9The kernel provides mitigation for such vulnerabilities in various
 10forms. Some of these mitigations are compile-time configurable and some
 11can be supplied on the kernel command line.
 12
 13There is also a class of mitigations which are very expensive, but they can
 14be restricted to a certain set of processes or tasks in controlled
 15environments. The mechanism to control these mitigations is via
 16:manpage:`prctl(2)`.
 17
 18There are two prctl options which are related to this:
 19
 20 * PR_GET_SPECULATION_CTRL
 21
 22 * PR_SET_SPECULATION_CTRL
 23
 24PR_GET_SPECULATION_CTRL
 25-----------------------
 26
 27PR_GET_SPECULATION_CTRL returns the state of the speculation misfeature
 28which is selected with arg2 of prctl(2). The return value uses bits 0-3 with
 29the following meaning:
 30
 31==== ====================== ==================================================
 32Bit  Define                 Description
 33==== ====================== ==================================================
 340    PR_SPEC_PRCTL          Mitigation can be controlled per task by
 35                            PR_SET_SPECULATION_CTRL.
 361    PR_SPEC_ENABLE         The speculation feature is enabled, mitigation is
 37                            disabled.
 382    PR_SPEC_DISABLE        The speculation feature is disabled, mitigation is
 39                            enabled.
 403    PR_SPEC_FORCE_DISABLE  Same as PR_SPEC_DISABLE, but cannot be undone. A
 41                            subsequent prctl(..., PR_SPEC_ENABLE) will fail.
 424    PR_SPEC_DISABLE_NOEXEC Same as PR_SPEC_DISABLE, but the state will be
 43                            cleared on :manpage:`execve(2)`.
 44==== ====================== ==================================================
 45
 46If all bits are 0 the CPU is not affected by the speculation misfeature.
 47
 48If PR_SPEC_PRCTL is set, then the per-task control of the mitigation is
 49available. If not set, prctl(PR_SET_SPECULATION_CTRL) for the speculation
 50misfeature will fail.
 51
 52.. _set_spec_ctrl:
 53
 54PR_SET_SPECULATION_CTRL
 55-----------------------
 56
 57PR_SET_SPECULATION_CTRL allows to control the speculation misfeature, which
 58is selected by arg2 of :manpage:`prctl(2)` per task. arg3 is used to hand
 59in the control value, i.e. either PR_SPEC_ENABLE or PR_SPEC_DISABLE or
 60PR_SPEC_FORCE_DISABLE.
 61
 62Common error codes
 63------------------
 64======= =================================================================
 65Value   Meaning
 66======= =================================================================
 67EINVAL  The prctl is not implemented by the architecture or unused
 68        prctl(2) arguments are not 0.
 69
 70ENODEV  arg2 is selecting a not supported speculation misfeature.
 71======= =================================================================
 72
 73PR_SET_SPECULATION_CTRL error codes
 74-----------------------------------
 75======= =================================================================
 76Value   Meaning
 77======= =================================================================
 780       Success
 79
 80ERANGE  arg3 is incorrect, i.e. it's neither PR_SPEC_ENABLE nor
 81        PR_SPEC_DISABLE nor PR_SPEC_FORCE_DISABLE.
 82
 83ENXIO   Control of the selected speculation misfeature is not possible.
 84        See PR_GET_SPECULATION_CTRL.
 85
 86EPERM   Speculation was disabled with PR_SPEC_FORCE_DISABLE and caller
 87        tried to enable it again.
 88======= =================================================================
 89
 90Speculation 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);
 98   * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_FORCE_DISABLE, 0, 0);
 99   * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_DISABLE_NOEXEC, 0, 0);
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);
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);