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-rw-r--r--Documentation/userspace-api/index.rst1
-rw-r--r--Documentation/userspace-api/spec_ctrl.rst86
2 files changed, 87 insertions, 0 deletions
diff --git a/Documentation/userspace-api/index.rst b/Documentation/userspace-api/index.rst
index 7b2eb1b7d4ca..a3233da7fa88 100644
--- a/Documentation/userspace-api/index.rst
+++ b/Documentation/userspace-api/index.rst
@@ -19,6 +19,7 @@ place where this information is gathered.
no_new_privs
seccomp_filter
unshare
+ spec_ctrl
.. only:: subproject and html
diff --git a/Documentation/userspace-api/spec_ctrl.rst b/Documentation/userspace-api/spec_ctrl.rst
new file mode 100644
index 000000000000..ddbebcd01208
--- /dev/null
+++ b/Documentation/userspace-api/spec_ctrl.rst
@@ -0,0 +1,86 @@
+===================
+Speculation Control
+===================
+
+Quite some CPUs have speculation related misfeatures which are in fact
+vulnerabilites causing data leaks in various forms even accross privilege
+domains.
+
+The kernel provides mitigation for such vulnerabilities in various
+forms. Some of these mitigations are compile time configurable and some on
+the kernel command line.
+
+There is also a class of mitigations which are very expensive, but they can
+be restricted to a certain set of processes or tasks in controlled
+environments. The mechanism to control these mitigations is via
+:manpage:`prctl(2)`.
+
+There are two prctl options which are related to this:
+
+ * PR_GET_SPECULATION_CTRL
+
+ * PR_SET_SPECULATION_CTRL
+
+PR_GET_SPECULATION_CTRL
+-----------------------
+
+PR_GET_SPECULATION_CTRL returns the state of the speculation misfeature
+which is selected with arg2 of prctl(2). The return value uses bits 0-2 with
+the following meaning:
+
+==== ================ ===================================================
+Bit Define Description
+==== ================ ===================================================
+0 PR_SPEC_PRCTL Mitigation can be controlled per task by
+ PR_SET_SPECULATION_CTRL
+1 PR_SPEC_ENABLE The speculation feature is enabled, mitigation is
+ disabled
+2 PR_SPEC_DISABLE The speculation feature is disabled, mitigation is
+ enabled
+==== ================ ===================================================
+
+If all bits are 0 the CPU is not affected by the speculation misfeature.
+
+If PR_SPEC_PRCTL is set, then the per task control of the mitigation is
+available. If not set, prctl(PR_SET_SPECULATION_CTRL) for the speculation
+misfeature will fail.
+
+PR_SET_SPECULATION_CTRL
+-----------------------
+PR_SET_SPECULATION_CTRL allows to control the speculation misfeature, which
+is selected by arg2 of :manpage:`prctl(2)` per task. arg3 is used to hand
+in the control value, i.e. either PR_SPEC_ENABLE or PR_SPEC_DISABLE.
+
+Common error codes
+------------------
+======= =================================================================
+Value Meaning
+======= =================================================================
+EINVAL The prctl is not implemented by the architecture or unused
+ prctl(2) arguments are not 0
+
+ENODEV arg2 is selecting a not supported speculation misfeature
+======= =================================================================
+
+PR_SET_SPECULATION_CTRL error codes
+-----------------------------------
+======= =================================================================
+Value Meaning
+======= =================================================================
+0 Success
+
+ERANGE arg3 is incorrect, i.e. it's neither PR_SPEC_ENABLE nor
+ PR_SPEC_DISABLE
+
+ENXIO Control of the selected speculation misfeature is not possible.
+ See PR_GET_SPECULATION_CTRL.
+======= =================================================================
+
+Speculation misfeature controls
+-------------------------------
+- PR_SPEC_STORE_BYPASS: Speculative Store Bypass
+
+ Invocations:
+ * prctl(PR_GET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, 0, 0, 0);
+ * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_ENABLE, 0, 0);
+ * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_DISABLE, 0, 0);