[RFC PATCH 2/3] x86/microcode: Default-disable late loading

[Borislav Petkov <bp@alien8.de>]

From: Borislav Petkov <bp@suse.de>

It is dangerous and it should not be used anyway - there's a nice early
loading already.

Requested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
---
 arch/x86/Kconfig                     | 11 +++++++++++
 arch/x86/kernel/cpu/common.c         |  2 ++
 arch/x86/kernel/cpu/microcode/core.c |  7 ++++++-
 3 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 1c0da2dbfb26..33891b82fb65 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1345,6 +1345,17 @@ config MICROCODE_AMD
 	  If you select this option, microcode patch loading support for AMD
 	  processors will be enabled.
 
+config MICROCODE_LATE_LOADING
+	bool "Late microcode loading (DANGEROUS)"
+	default n
+	depends on MICROCODE
+	help
+	  Loading microcode late, when the system is up and executing instructions
+	  is a tricky business and should be avoided if possible. Just the sequence
+	  of synchronizing all cores and SMT threads is one fragile dance which does
+	  not guarantee that cores might not softlock after the loading. Therefore,
+	  use this at your own risk. Late loading taints the kernel too.
+
 config X86_MSR
 	tristate "/dev/cpu/*/msr - Model-specific register support"
 	help
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 2e9142797c99..c296cb1c0113 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/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
diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
index b72c4134f289..c717db6b6856 100644
--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/arch/x86/kernel/cpu/microcode/core.c
@@ -376,6 +376,7 @@ static int apply_microcode_on_target(int cpu)
 /* 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?
  *
@@ -543,6 +544,9 @@ static ssize_t reload_store(struct device *dev,
 	return ret;
 }
 
+static DEVICE_ATTR_WO(reload);
+#endif
+
 static ssize_t version_show(struct device *dev,
 			struct device_attribute *attr, char *buf)
 {
@@ -559,7 +563,6 @@ 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);
 
@@ -712,7 +715,9 @@ 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
 };
 
-- 
2.35.1