[PATCH v1 Part2 5/5] x86/microcode: Provide an option to override minrev enforcement

[Ashok Raj]

Minimum Required Revision (minrev) is enforced strictly. All new patches
will have a minrev that is not zero. But there might be a transition time
for some that need this enforcement to be relaxed.

When the override is enabled, the kernel will be tainted.

Provide a debugfs variable to override the minrev enforcement.

Signed-off-by: Ashok Raj <ashok.raj@xxxxxxxxx>
Cc: LKML <linux-kernel@xxxxxxxxxxxxxxx>
Cc: x86 <x86@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Tony Luck <tony.luck@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: Alison Schofield <alison.schofield@xxxxxxxxx>
Cc: Reinette Chatre <reinette.chatre@xxxxxxxxx>
Cc: Thomas Gleixner (Intel) <tglx@xxxxxxxxxxxxx>
Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
Cc: Stefan Talpalaru <stefantalpalaru@xxxxxxxxx>
Cc: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: Rafael J. Wysocki <rafael@xxxxxxxxxx>
Cc: Peter Zilstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
---
This patch is optional.
---
 arch/x86/include/asm/microcode.h      |  2 ++
 arch/x86/kernel/cpu/microcode/core.c  | 13 ++++++++++++-
 arch/x86/kernel/cpu/microcode/intel.c |  8 ++++++++
 3 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/microcode.h b/arch/x86/include/asm/microcode.h
index 3d48143e84a9..d82f22d50ebd 100644
--- a/arch/x86/include/asm/microcode.h
+++ b/arch/x86/include/asm/microcode.h
@@ -16,6 +16,8 @@ struct ucode_patch {
 
 extern struct list_head microcode_cache;
 
+extern bool override_minrev;
+
 struct cpu_signature {
 	unsigned int sig;
 	unsigned int pf;
diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
index 446ddf3fcc29..5ed60c6c8e8d 100644
--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/arch/x86/kernel/cpu/microcode/core.c
@@ -24,6 +24,7 @@
 #include <linux/miscdevice.h>
 #include <linux/capability.h>
 #include <linux/firmware.h>
+#include <linux/debugfs.h>
 #include <linux/kernel.h>
 #include <linux/delay.h>
 #include <linux/mutex.h>
@@ -44,7 +45,9 @@
 #define DRIVER_VERSION	"2.2"
 
 static struct microcode_ops	*microcode_ops;
+static struct dentry		*dentry_ucode;
 static bool dis_ucode_ldr = true;
+bool override_minrev;
 
 bool initrd_gone;
 
@@ -497,7 +500,11 @@ static ssize_t reload_store(struct device *dev,
 		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");
 		ret = -EINVAL;
-		goto put;
+
+		if (!override_minrev)
+			goto put;
+
+		pr_err("Overriding minrev\n");
 	}
 
 	tmp_ret = microcode_ops->request_microcode_fw(bsp, &microcode_pdev->dev);
@@ -688,7 +695,11 @@ static int __init microcode_init(void)
 	cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN, "x86/microcode:online",
 				  mc_cpu_online, mc_cpu_down_prep);
 
+	dentry_ucode = debugfs_create_dir("microcode", NULL);
+	debugfs_create_bool("override_minrev", 0644, dentry_ucode, &override_minrev);
+
 	pr_info("Microcode Update Driver: v%s.", DRIVER_VERSION);
+	pr_info("Override minrev %s\n", override_minrev ? "enabled" : "disabled");
 
 	return 0;
 
diff --git a/arch/x86/kernel/cpu/microcode/intel.c b/arch/x86/kernel/cpu/microcode/intel.c
index 68a3c5569cd2..172e1f166844 100644
--- a/arch/x86/kernel/cpu/microcode/intel.c
+++ b/arch/x86/kernel/cpu/microcode/intel.c
@@ -141,6 +141,14 @@ static int is_lateload_safe(struct microcode_header_intel *mc_header)
 {
 	struct ucode_cpu_info uci;
 
+	/*
+	 * If minrev is bypassed via debugfs, then allow late-load.
+	 */
+	if (override_minrev) {
+		pr_info("Bypassing minrev enforcement via debugfs\n");
+		return 0;
+	}
+
 	/*
 	 * When late-loading, ensure the header declares a minimum revision
 	 * required to perform a late-load.
-- 
2.34.1