X86 microcode updates:

- Disable late microcode loading by default. Unless the HW people get
     their act together and provide a required minimum version in the
     microcode header for making a halfways informed decision its just
     lottery and broken.
 
   - Warn and taint the kernel when microcode is loaded late
 
   - Remove the old unused microcode loader interface
 
   - Remove a redundant perf callback from the microcode loader
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Merge tag 'x86-microcode-2022-06-05' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 microcode updates from Thomas Gleixner:

 - Disable late microcode loading by default. Unless the HW people get
   their act together and provide a required minimum version in the
   microcode header for making a halfways informed decision its just
   lottery and broken.

 - Warn and taint the kernel when microcode is loaded late

 - Remove the old unused microcode loader interface

 - Remove a redundant perf callback from the microcode loader

* tag 'x86-microcode-2022-06-05' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/microcode: Remove unnecessary perf callback
  x86/microcode: Taint and warn on late loading
  x86/microcode: Default-disable late loading
  x86/microcode: Rip out the OLD_INTERFACE

arch/x86/Kconfig

@@ -1358,17 +1358,16 @@ config MICROCODE_AMD
 	  If you select this option, microcode patch loading support for AMD
 	  processors will be enabled.
 
-config MICROCODE_OLD_INTERFACE
+config MICROCODE_LATE_LOADING
-	bool "Ancient loading interface (DEPRECATED)"
+	bool "Late microcode loading (DANGEROUS)"
 	default n
 	depends on MICROCODE
 	help
-	  DO NOT USE THIS! This is the ancient /dev/cpu/microcode interface
+	  Loading microcode late, when the system is up and executing instructions
-	  which was used by userspace tools like iucode_tool and microcode.ctl.
+	  is a tricky business and should be avoided if possible. Just the sequence
-	  It is inadequate because it runs too late to be able to properly
+	  of synchronizing all cores and SMT threads is one fragile dance which does
-	  load microcode on a machine and it needs special tools. Instead, you
+	  not guarantee that cores might not softlock after the loading. Therefore,
-	  should've switched to the early loading method with the initrd or
+	  use this at your own risk. Late loading taints the kernel too.
-	  builtin microcode by now: Documentation/x86/microcode.rst
 
 config X86_MSR
 	tristate "/dev/cpu/*/msr - Model-specific register support"

arch/x86/kernel/cpu/common.c

@@ -2222,6 +2222,7 @@ void cpu_init_secondary(void)
 }
 #endif
 
+#ifdef CONFIG_MICROCODE_LATE_LOADING
 /*
  * The microcode loader calls this upon late microcode load to recheck features,
  * only when microcode has been updated. Caller holds microcode_mutex and CPU
@@ -2251,6 +2252,7 @@ void microcode_check(void)
 	pr_warn("x86/CPU: CPU features have changed after loading microcode, but might not take effect.\n");
 	pr_warn("x86/CPU: Please consider either early loading through initrd/built-in or a potential BIOS update.\n");
 }
+#endif
 
 /*
  * Invoked from core CPU hotplug code after hotplug operations

arch/x86/kernel/cpu/microcode/core.c

@@ -373,101 +373,10 @@ static int apply_microcode_on_target(int cpu)
 	return ret;
 }
 
-#ifdef CONFIG_MICROCODE_OLD_INTERFACE
-static int do_microcode_update(const void __user *buf, size_t size)
-{
-	int error = 0;
-	int cpu;
-
-	for_each_online_cpu(cpu) {
-		struct ucode_cpu_info *uci = ucode_cpu_info + cpu;
-		enum ucode_state ustate;
-
-		if (!uci->valid)
-			continue;
-
-		ustate = microcode_ops->request_microcode_user(cpu, buf, size);
-		if (ustate == UCODE_ERROR) {
-			error = -1;
-			break;
-		} else if (ustate == UCODE_NEW) {
-			apply_microcode_on_target(cpu);
-		}
-	}
-
-	return error;
-}
-
-static int microcode_open(struct inode *inode, struct file *file)
-{
-	return capable(CAP_SYS_RAWIO) ? stream_open(inode, file) : -EPERM;
-}
-
-static ssize_t microcode_write(struct file *file, const char __user *buf,
-			       size_t len, loff_t *ppos)
-{
-	ssize_t ret = -EINVAL;
-	unsigned long nr_pages = totalram_pages();
-
-	if ((len >> PAGE_SHIFT) > nr_pages) {
-		pr_err("too much data (max %ld pages)\n", nr_pages);
-		return ret;
-	}
-
-	cpus_read_lock();
-	mutex_lock(&microcode_mutex);
-
-	if (do_microcode_update(buf, len) == 0)
-		ret = (ssize_t)len;
-
-	if (ret > 0)
-		perf_check_microcode();
-
-	mutex_unlock(&microcode_mutex);
-	cpus_read_unlock();
-
-	return ret;
-}
-
-static const struct file_operations microcode_fops = {
-	.owner			= THIS_MODULE,
-	.write			= microcode_write,
-	.open			= microcode_open,
-	.llseek		= no_llseek,
-};
-
-static struct miscdevice microcode_dev = {
-	.minor			= MICROCODE_MINOR,
-	.name			= "microcode",
-	.nodename		= "cpu/microcode",
-	.fops			= &microcode_fops,
-};
-
-static int __init microcode_dev_init(void)
-{
-	int error;
-
-	error = misc_register(&microcode_dev);
-	if (error) {
-		pr_err("can't misc_register on minor=%d\n", MICROCODE_MINOR);
-		return error;
-	}
-
-	return 0;
-}
-
-static void __exit microcode_dev_exit(void)
-{
-	misc_deregister(&microcode_dev);
-}
-#else
-#define microcode_dev_init()	0
-#define microcode_dev_exit()	do { } while (0)
-#endif
-
 /* fake device for request_firmware */
 static struct platform_device	*microcode_pdev;
 
+#ifdef CONFIG_MICROCODE_LATE_LOADING
 /*
  * Late loading dance. Why the heavy-handed stomp_machine effort?
  *
@@ -584,6 +493,9 @@ static int microcode_reload_late(void)
 {
 	int ret;
 
+	pr_err("Attempting late microcode loading - it is dangerous and taints the kernel.\n");
+	pr_err("You should switch to early loading, if possible.\n");
+
 	atomic_set(&late_cpus_in,  0);
 	atomic_set(&late_cpus_out, 0);
 
@@ -632,9 +544,14 @@ put:
 	if (ret == 0)
 		ret = size;
 
+	add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
+
 	return ret;
 }
 
+static DEVICE_ATTR_WO(reload);
+#endif
+
 static ssize_t version_show(struct device *dev,
 			struct device_attribute *attr, char *buf)
 {
@@ -651,7 +568,6 @@ static ssize_t pf_show(struct device *dev,
 	return sprintf(buf, "0x%x\n", uci->cpu_sig.pf);
 }
 
-static DEVICE_ATTR_WO(reload);
 static DEVICE_ATTR(version, 0444, version_show, NULL);
 static DEVICE_ATTR(processor_flags, 0444, pf_show, NULL);
 
@@ -804,7 +720,9 @@ static int mc_cpu_down_prep(unsigned int cpu)
 }
 
 static struct attribute *cpu_root_microcode_attrs[] = {
+#ifdef CONFIG_MICROCODE_LATE_LOADING
 	&dev_attr_reload.attr,
+#endif
 	NULL
 };
 
@@ -838,10 +756,7 @@ static int __init microcode_init(void)
 
 	cpus_read_lock();
 	mutex_lock(&microcode_mutex);
-
 	error = subsys_interface_register(&mc_cpu_interface);
-	if (!error)
-		perf_check_microcode();
 	mutex_unlock(&microcode_mutex);
 	cpus_read_unlock();
 
@@ -856,10 +771,6 @@ static int __init microcode_init(void)
 		goto out_driver;
 	}
 
-	error = microcode_dev_init();
-	if (error)
-		goto out_ucode_group;
-
 	register_syscore_ops(&mc_syscore_ops);
 	cpuhp_setup_state_nocalls(CPUHP_AP_MICROCODE_LOADER, "x86/microcode:starting",
 				  mc_cpu_starting, NULL);
@@ -870,10 +781,6 @@ static int __init microcode_init(void)
 
 	return 0;
 
- out_ucode_group:
-	sysfs_remove_group(&cpu_subsys.dev_root->kobj,
-			   &cpu_root_microcode_group);
-
  out_driver:
 	cpus_read_lock();
 	mutex_lock(&microcode_mutex);