prctl: Add force disable speculation

For certain use cases it is desired to enforce mitigations so they cannot
be undone afterwards. That's important for loader stubs which want to
prevent a child from disabling the mitigation again. Will also be used for
seccomp(). The extra state preserving of the prctl state for SSB is a
preparatory step for EBPF dymanic speculation control.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 21 insertions, 13 deletions

diff --git a/Documentation/userspace-api/spec_ctrl.rst b/Documentation/userspace-api/spec_ctrl.rst
index ddbebcd01208..1b3690d30943 100644
--- a/Documentation/userspace-api/spec_ctrl.rst
+++ b/Documentation/userspace-api/spec_ctrl.rst
@@ -25,19 +25,21 @@ 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
+which is selected with arg2 of prctl(2). The return value uses bits 0-3 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
-==== ================ ===================================================
+==== ===================== ===================================================
+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
+3    PR_SPEC_FORCE_DISABLE Same as PR_SPEC_DISABLE, but cannot be undone. A
+                           subsequent prctl(..., PR_SPEC_ENABLE) will fail.
+==== ===================== ===================================================
 
 If all bits are 0 the CPU is not affected by the speculation misfeature.
 
@@ -47,9 +49,11 @@ 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.
+in the control value, i.e. either PR_SPEC_ENABLE or PR_SPEC_DISABLE or
+PR_SPEC_FORCE_DISABLE.
 
 Common error codes
 ------------------
@@ -70,10 +74,13 @@ Value   Meaning
 0       Success
 
 ERANGE  arg3 is incorrect, i.e. it's neither PR_SPEC_ENABLE nor
-        PR_SPEC_DISABLE
+        PR_SPEC_DISABLE nor PR_SPEC_FORCE_DISABLE
 
 ENXIO   Control of the selected speculation misfeature is not possible.
         See PR_GET_SPECULATION_CTRL.
+
+EPERM   Speculation was disabled with PR_SPEC_FORCE_DISABLE and caller
+        tried to enable it again.
 ======= =================================================================
 
 Speculation misfeature controls
@@ -84,3 +91,4 @@ Speculation misfeature controls
    * 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);
+   * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_FORCE_DISABLE, 0, 0);