[Patch v4 16/18] x86/speculation: Enable STIBP to protect security sensitive tasks

[Tim Chen <tim.c.chen@linux.intel.com>]

Enable STIBP defense on high security tasks.

For normal tasks, STIBP is unused so they are not impacted by overhead
from STIBP in lite protection mode.

Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
---
 arch/x86/kernel/cpu/bugs.c | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 54f4675..b402b96 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -14,6 +14,8 @@
 #include <linux/module.h>
 #include <linux/nospec.h>
 #include <linux/prctl.h>
+#include <linux/sched/coredump.h>
+#include <linux/security.h>
 
 #include <asm/spec-ctrl.h>
 #include <asm/cmdline.h>
@@ -770,6 +772,37 @@ static int ssb_prctl_set(struct task_struct *task, unsigned long ctrl)
 	return 0;
 }
 
+static void set_task_stibp(struct task_struct *tsk, bool stibp_on)
+{
+	bool update = false;
+
+	if (!static_branch_unlikely(&spectre_v2_app_lite))
+		return;
+
+	if (stibp_on)
+		update = !test_and_set_tsk_thread_flag(tsk, TIF_STIBP);
+	else
+		update = test_and_clear_tsk_thread_flag(tsk, TIF_STIBP);
+
+	if (!update)
+		return;
+
+	if (tsk == current)
+		speculation_ctrl_update_current();
+}
+
+void arch_set_security(struct task_struct *tsk, unsigned int value)
+{
+	if (value > SECURITY_HIGH)
+		return;
+
+	/* Update STIBP defenses */
+	if (value == SECURITY_HIGH)
+		set_task_stibp(tsk, true);
+	else
+		set_task_stibp(tsk, false);
+}
+
 int arch_prctl_spec_ctrl_set(struct task_struct *task, unsigned long which,
 			     unsigned long ctrl)
 {
-- 
2.9.4