[PATCH 2/2] x86, microcode: Add a disable chicken bit

From: Borislav Petkov
Date: Mon May 19 2014 - 14:59:48 EST


From: Borislav Petkov <bp@xxxxxxx>

Add a cmdline param which disables the microcode loader. This is useful
mostly in debugging situations where we want to turn off microcode
loading, both early from the initrd and late, as a means to be able to
rule out its influence on the machine.

Signed-off-by: Borislav Petkov <bp@xxxxxxx>
---
arch/x86/include/asm/microcode.h | 1 +
arch/x86/kernel/cpu/microcode/core.c | 6 +++++
arch/x86/kernel/cpu/microcode/core_early.c | 37 ++++++++++++++++++++++++++++++
3 files changed, 44 insertions(+)

diff --git a/arch/x86/include/asm/microcode.h b/arch/x86/include/asm/microcode.h
index b59827e76529..64dc362506b7 100644
--- a/arch/x86/include/asm/microcode.h
+++ b/arch/x86/include/asm/microcode.h
@@ -25,6 +25,7 @@ struct cpu_signature {
struct device;

enum ucode_state { UCODE_ERROR, UCODE_OK, UCODE_NFOUND };
+extern bool dis_ucode_ldr;

struct microcode_ops {
enum ucode_state (*request_microcode_user) (int cpu,
diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
index 15c987698b0f..dd9d6190b08d 100644
--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/arch/x86/kernel/cpu/microcode/core.c
@@ -97,6 +97,9 @@ MODULE_LICENSE("GPL");

static struct microcode_ops *microcode_ops;

+bool dis_ucode_ldr;
+module_param(dis_ucode_ldr, bool, 0);
+
/*
* Synchronization.
*
@@ -546,6 +549,9 @@ static int __init microcode_init(void)
struct cpuinfo_x86 *c = &cpu_data(0);
int error;

+ if (dis_ucode_ldr)
+ return 0;
+
if (c->x86_vendor == X86_VENDOR_INTEL)
microcode_ops = init_intel_microcode();
else if (c->x86_vendor == X86_VENDOR_AMD)
diff --git a/arch/x86/kernel/cpu/microcode/core_early.c b/arch/x86/kernel/cpu/microcode/core_early.c
index be7f8514f577..5f28a64e71ea 100644
--- a/arch/x86/kernel/cpu/microcode/core_early.c
+++ b/arch/x86/kernel/cpu/microcode/core_early.c
@@ -17,9 +17,11 @@
* 2 of the License, or (at your option) any later version.
*/
#include <linux/module.h>
+#include <asm/microcode.h>
#include <asm/microcode_intel.h>
#include <asm/microcode_amd.h>
#include <asm/processor.h>
+#include <asm/cmdline.h>

#define QCHAR(a, b, c, d) ((a) + ((b) << 8) + ((c) << 16) + ((d) << 24))
#define CPUID_INTEL1 QCHAR('G', 'e', 'n', 'u')
@@ -72,10 +74,33 @@ static int x86_family(void)
return x86;
}

+static bool __init check_loader_disabled_bsp(void)
+{
+#ifdef CONFIG_X86_32
+ const char *cmdline = (const char *)__pa_nodebug(boot_command_line);
+ const char *opt = "dis_ucode_ldr";
+ const char *option = (const char *)__pa_nodebug(opt);
+ bool *res = (bool *)__pa_nodebug(&dis_ucode_ldr);
+
+#else /* CONFIG_X86_64 */
+ const char *cmdline = boot_command_line;
+ const char *option = "dis_ucode_ldr";
+ bool *res = &dis_ucode_ldr;
+#endif
+
+ if (cmdline_find_option_bool(cmdline, option))
+ *res = true;
+
+ return *res;
+}
+
void __init load_ucode_bsp(void)
{
int vendor, x86;

+ if (check_loader_disabled_bsp())
+ return;
+
if (!have_cpuid_p())
return;

@@ -96,10 +121,22 @@ void __init load_ucode_bsp(void)
}
}

+static bool check_loader_disabled_ap(void)
+{
+#ifdef CONFIG_X86_32
+ return __pa_nodebug(dis_ucode_ldr);
+#else
+ return dis_ucode_ldr;
+#endif
+}
+
void load_ucode_ap(void)
{
int vendor, x86;

+ if (check_loader_disabled_ap())
+ return;
+
if (!have_cpuid_p())
return;

--
1.9.0

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