UBUNTU: SAUCE: x86/mce: register mce notifier earlier

Currently the hotplug notifier is registered in device init. This is
too late to handle events at early SMP boot stage. Later upstream code
(around 4.10) changes this, but relies on the work done to make the
whole CPU hotplug code a state machine.

CVE-2018-3620
CVE-2018-3646
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

arch/x86/kernel/cpu/mcheck/mce.c

@@ -1680,6 +1680,9 @@ dotraplinkage void do_mce(struct pt_regs *regs, long error_code)
 	machine_check_vector(regs, error_code);
 }
 
+static struct notifier_block mce_cpu_notifier;
+static bool mce_cpu_notifier_registered = false;
+
 /*
  * Called for each booted CPU to set up machine checks.
  * Must be called with preempt off:
@@ -1710,6 +1713,18 @@ void mcheck_cpu_init(struct cpuinfo_x86 *c)
 
 	__mcheck_cpu_init_generic();
 	__mcheck_cpu_init_vendor(c);
+
+	if (!mce_cpu_notifier_registered) {
+		/*
+		 * Register the CPU hotplug notifier early (and it will
+		 * not be unregistered (as before) to not leave timers
+		 * undeleted. This function is called with required locks
+		 * being hold.
+		 */
+		__register_hotcpu_notifier(&mce_cpu_notifier);
+		mce_cpu_notifier_registered = true;
+	}
+
 	__mcheck_cpu_init_timer();
 }
 
@@ -2318,6 +2333,7 @@ static struct device_attribute *mce_device_attrs[] = {
 	NULL
 };
 
+static bool mce_device_initdone = false;
 static cpumask_var_t mce_device_initialized;
 
 static void mce_device_release(struct device *dev)
@@ -2434,14 +2450,16 @@ mce_cpu_callback(struct notifier_block *nfb, unsigned long action, void *hcpu)
 
 	switch (action & ~CPU_TASKS_FROZEN) {
 	case CPU_ONLINE:
-		mce_device_create(cpu);
+		if (mce_device_initdone)
+			mce_device_create(cpu);
 		if (threshold_cpu_callback)
 			threshold_cpu_callback(action, cpu);
 		break;
 	case CPU_DEAD:
 		if (threshold_cpu_callback)
 			threshold_cpu_callback(action, cpu);
-		mce_device_remove(cpu);
+		if (mce_device_initdone)
+			mce_device_remove(cpu);
 		mce_intel_hcpu_update(cpu);
 
 		/* intentionally ignoring frozen here */
@@ -2504,23 +2522,15 @@ static __init int mcheck_init_device(void)
 	if (err)
 		goto err_out_mem;
 
-	cpu_notifier_register_begin();
+	cpu_maps_update_begin();
 	for_each_online_cpu(i) {
 		err = mce_device_create(i);
 		if (err) {
-			/*
-			 * Register notifier anyway (and do not unreg it) so
-			 * that we don't leave undeleted timers, see notifier
-			 * callback above.
-			 */
-			__register_hotcpu_notifier(&mce_cpu_notifier);
-			cpu_notifier_register_done();
+			cpu_maps_update_done();
 			goto err_device_create;
 		}
 	}
-
-	__register_hotcpu_notifier(&mce_cpu_notifier);
-	cpu_notifier_register_done();
+	cpu_maps_update_done();
 
 	register_syscore_ops(&mce_syscore_ops);
 
@@ -2529,6 +2539,8 @@ static __init int mcheck_init_device(void)
 	if (err)
 		goto err_register;
 
+	mce_device_initdone = true;
+
 	return 0;
 
 err_register: