devcoredump: provide a one-way disable function

Since device/firmware coredumps can contain private data, it can
be desirable to turn them off unconditionally to be certain that
no such data will be collected by the system.

To achieve this, provide a "disabled" sysfs class attribute that
can only be changed from 0 to 1 and not back. Upon disabling,
discard existing coredumps and stop storing new ones.

Signed-off-by: Johannes Berg <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/base/devcoredump.c

···
 #include <linux/fs.h>
 #include <linux/workqueue.h>
 
+static struct class devcd_class;
+
+/* global disable flag, for security purposes */
+static bool devcd_disabled;
+
 /* if data isn't read by userspace after 5 minutes then delete it */
 #define DEVCD_TIMEOUT	(HZ * 60 * 5)
 
···
 	&devcd_dev_group, NULL,
 };
 
+static int devcd_free(struct device *dev, void *data)
+{
+	struct devcd_entry *devcd = dev_to_devcd(dev);
+
+	flush_delayed_work(&devcd->del_wk);
+	return 0;
+}
+
+static ssize_t disabled_show(struct class *class, struct class_attribute *attr,
+			     char *buf)
+{
+	return sprintf(buf, "%d\n", devcd_disabled);
+}
+
+static ssize_t disabled_store(struct class *class, struct class_attribute *attr,
+			      const char *buf, size_t count)
+{
+	long tmp = simple_strtol(buf, NULL, 10);
+
+	/*
+	 * This essentially makes the attribute write-once, since you can't
+	 * go back to not having it disabled. This is intentional, it serves
+	 * as a system lockdown feature.
+	 */
+	if (tmp != 1)
+		return -EINVAL;
+
+	devcd_disabled = true;
+
+	class_for_each_device(&devcd_class, NULL, NULL, devcd_free);
+
+	return count;
+}
+
+static struct class_attribute devcd_class_attrs[] = {
+	__ATTR_RW(disabled),
+	__ATTR_NULL
+};
+
 static struct class devcd_class = {
 	.name		= "devcoredump",
 	.owner		= THIS_MODULE,
 	.dev_release	= devcd_dev_release,
 	.dev_groups	= devcd_dev_groups,
+	.class_attrs	= devcd_class_attrs,
 };
 
 static ssize_t devcd_readv(char *buffer, loff_t offset, size_t count,
···
 	struct devcd_entry *devcd;
 	struct device *existing;
 
+	if (devcd_disabled)
+		goto free;
+
 	existing = class_find_device(&devcd_class, NULL, dev,
 				     devcd_match_failing);
 	if (existing) {
···
 	return class_register(&devcd_class);
 }
 __initcall(devcoredump_init);
-
-static int devcd_free(struct device *dev, void *data)
-{
-	struct devcd_entry *devcd = dev_to_devcd(dev);
-
-	flush_delayed_work(&devcd->del_wk);
-	return 0;
-}
 
 static void __exit devcoredump_exit(void)
 {