cpu/hotplug: Online siblings when SMT control is turned on

Writing 'off' to /sys/devices/system/cpu/smt/control offlines all SMT
siblings. Writing 'on' merily enables the abilify to online them, but does
not online them automatically.

Make 'on' more useful by onlining all offline siblings.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

CVE-2018-3620
CVE-2018-3646

[smb: _cpu_up() only has 2 arguments]
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

kernel/cpu.c

@@ -798,6 +798,15 @@ static void cpuhp_offline_cpu_device(unsigned int cpu)
 	kobject_uevent(&dev->kobj, KOBJ_OFFLINE);
 }
 
+static void cpuhp_online_cpu_device(unsigned int cpu)
+{
+	struct device *dev = get_cpu_device(cpu);
+
+	dev->offline = false;
+	/* Tell user space about the state change */
+	kobject_uevent(&dev->kobj, KOBJ_ONLINE);
+}
+
 static int cpuhp_smt_disable(enum cpuhp_smt_control ctrlval)
 {
 	int cpu, ret = 0;
@@ -830,11 +839,24 @@ static int cpuhp_smt_disable(enum cpuhp_smt_control ctrlval)
 	return ret;
 }
 
-static void cpuhp_smt_enable(void)
+static int cpuhp_smt_enable(void)
 {
+	int cpu, ret = 0;
+
 	cpu_maps_update_begin();
 	cpu_smt_control = CPU_SMT_ENABLED;
+	for_each_present_cpu(cpu) {
+		/* Skip online CPUs and CPUs on offline nodes */
+		if (cpu_online(cpu) || !node_online(cpu_to_node(cpu)))
+			continue;
+		ret = _cpu_up(cpu, 0);
+		if (ret)
+			break;
+		/* See comment in cpuhp_smt_disable() */
+		cpuhp_online_cpu_device(cpu);
+	}
 	cpu_maps_update_done();
+	return ret;
 }
 
 static ssize_t
@@ -865,7 +887,7 @@ store_smt_control(struct device *dev, struct device_attribute *attr,
 	if (ctrlval != cpu_smt_control) {
 		switch (ctrlval) {
 		case CPU_SMT_ENABLED:
-			cpuhp_smt_enable();
+			ret = cpuhp_smt_enable();
 			break;
 		case CPU_SMT_DISABLED:
 		case CPU_SMT_FORCE_DISABLED: