ACPI: Split out custom_method functionality into an own driver

With /sys/kernel/debug/acpi/custom_method root can write
to arbitrary memory and increase his priveleges, even if
these are restricted.

-> Make this an own debug .config option and warn about the
security issue in the config description.

-> Still keep acpi/debugfs.c which now only creates an empty
   /sys/kernel/debug/acpi directory. There might be other
   users of it later.
Signed-off-by: Thomas Renninger <trenn@suse.de>
Acked-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: rui.zhang@intel.com
Signed-off-by: Len Brown <len.brown@intel.com>

Documentation/acpi/method-customizing.txt

@@ -66,3 +66,8 @@ Note: We can use a kernel with multiple custom ACPI method running,
       But each individual write to debugfs can implement a SINGLE
       method override. i.e. if we want to insert/override multiple
       ACPI methods, we need to redo step c) ~ g) for multiple times.
+
+Note: Be aware that root can mis-use this driver to modify arbitrary
+      memory and gain additional rights, if root's privileges got
+      restricted (for example if root is not allowed to load additional
+      modules after boot).

drivers/acpi/Kconfig

@@ -380,6 +380,21 @@ config ACPI_HED
 	  which is used to report some hardware errors notified via
 	  SCI, mainly the corrected errors.
 
+config ACPI_CUSTOM_METHOD
+	tristate "Allow ACPI methods to be inserted/replaced at run time"
+	depends on DEBUG_FS
+	default n
+	help
+	  This debug facility allows ACPI AML methods to me inserted and/or
+	  replaced without rebooting the system. For details refer to:
+	  Documentation/acpi/method-customizing.txt.
+
+	  NOTE: This option is security sensitive, because it allows arbitrary
+	  kernel memory to be written to by root (uid=0) users, allowing them
+	  to bypass certain security measures (e.g. if root is not allowed to
+	  load additional kernel modules after boot, this feature may be used
+	  to override that restriction).
+
 source "drivers/acpi/apei/Kconfig"
 
 endif	# ACPI

drivers/acpi/Makefile

@@ -62,6 +62,7 @@ obj-$(CONFIG_ACPI_SBS)		+= sbs.o
 obj-$(CONFIG_ACPI_POWER_METER)	+= power_meter.o
 obj-$(CONFIG_ACPI_HED)		+= hed.o
 obj-$(CONFIG_ACPI_EC_DEBUGFS)	+= ec_sys.o
+obj-$(CONFIG_ACPI_CUSTOM_METHOD)+= custom_method.o
 
 # processor has its own "processor." module_param namespace
 processor-y			:= processor_driver.o processor_throttling.o

drivers/acpi/custom_method.c

+/*
+ * debugfs.c - ACPI debugfs interface to userspace.
+ */
+
+#include <linux/init.h>
+#include <linux/module.h>
+#include <linux/kernel.h>
+#include <linux/uaccess.h>
+#include <linux/debugfs.h>
+#include <acpi/acpi_drivers.h>
+
+#include "internal.h"
+
+#define _COMPONENT		ACPI_SYSTEM_COMPONENT
+ACPI_MODULE_NAME("custom_method");
+MODULE_LICENSE("GPL");
+
+static struct dentry *cm_dentry;
+
+/* /sys/kernel/debug/acpi/custom_method */
+
+static ssize_t cm_write(struct file *file, const char __user * user_buf,
+			size_t count, loff_t *ppos)
+{
+	static char *buf;
+	static u32 max_size;
+	static u32 uncopied_bytes;
+
+	struct acpi_table_header table;
+	acpi_status status;
+
+	if (!(*ppos)) {
+		/* parse the table header to get the table length */
+		if (count <= sizeof(struct acpi_table_header))
+			return -EINVAL;
+		if (copy_from_user(&table, user_buf,
+				   sizeof(struct acpi_table_header)))
+			return -EFAULT;
+		uncopied_bytes = max_size = table.length;
+		buf = kzalloc(max_size, GFP_KERNEL);
+		if (!buf)
+			return -ENOMEM;
+	}
+
+	if (buf == NULL)
+		return -EINVAL;
+
+	if ((*ppos > max_size) ||
+	    (*ppos + count > max_size) ||
+	    (*ppos + count < count) ||
+	    (count > uncopied_bytes))
+		return -EINVAL;
+
+	if (copy_from_user(buf + (*ppos), user_buf, count)) {
+		kfree(buf);
+		buf = NULL;
+		return -EFAULT;
+	}
+
+	uncopied_bytes -= count;
+	*ppos += count;
+
+	if (!uncopied_bytes) {
+		status = acpi_install_method(buf);
+		kfree(buf);
+		buf = NULL;
+		if (ACPI_FAILURE(status))
+			return -EINVAL;
+		add_taint(TAINT_OVERRIDDEN_ACPI_TABLE);
+	}
+
+	return count;
+}
+
+static const struct file_operations cm_fops = {
+	.write = cm_write,
+	.llseek = default_llseek,
+};
+
+static int __init acpi_custom_method_init(void)
+{
+	if (acpi_debugfs_dir == NULL)
+		return -ENOENT;
+
+	cm_dentry = debugfs_create_file("custom_method", S_IWUSR,
+					acpi_debugfs_dir, NULL, &cm_fops);
+	if (cm_dentry == NULL)
+		return -ENODEV;
+
+	return 0;
+}
+
+static void __exit acpi_custom_method_exit(void)
+{
+	if (cm_dentry)
+		debugfs_remove(cm_dentry);
+ }
+
+module_init(acpi_custom_method_init);
+module_exit(acpi_custom_method_exit);

drivers/acpi/debugfs.c

@@ -3,9 +3,6 @@
  */
 
 #include <linux/init.h>
-#include <linux/module.h>
-#include <linux/kernel.h>
-#include <linux/uaccess.h>
 #include <linux/debugfs.h>
 #include <acpi/acpi_drivers.h>
 
@@ -13,84 +10,9 @@
 ACPI_MODULE_NAME("debugfs");
 
 struct dentry *acpi_debugfs_dir;
-static struct dentry *cm_dentry;
-
-/* /sys/kernel/debug/acpi/custom_method */
-
-static ssize_t cm_write(struct file *file, const char __user * user_buf,
-			size_t count, loff_t *ppos)
-{
-	static char *buf;
-	static u32 max_size;
-	static u32 uncopied_bytes;
-
-	struct acpi_table_header table;
-	acpi_status status;
-
-	if (!(*ppos)) {
-		/* parse the table header to get the table length */
-		if (count <= sizeof(struct acpi_table_header))
-			return -EINVAL;
-		if (copy_from_user(&table, user_buf,
-				   sizeof(struct acpi_table_header)))
-			return -EFAULT;
-		uncopied_bytes = max_size = table.length;
-		buf = kzalloc(max_size, GFP_KERNEL);
-		if (!buf)
-			return -ENOMEM;
-	}
-
-	if (buf == NULL)
-		return -EINVAL;
-
-	if ((*ppos > max_size) ||
-	    (*ppos + count > max_size) ||
-	    (*ppos + count < count) ||
-	    (count > uncopied_bytes))
-		return -EINVAL;
-
-	if (copy_from_user(buf + (*ppos), user_buf, count)) {
-		kfree(buf);
-		buf = NULL;
-		return -EFAULT;
-	}
-
-	uncopied_bytes -= count;
-	*ppos += count;
-
-	if (!uncopied_bytes) {
-		status = acpi_install_method(buf);
-		kfree(buf);
-		buf = NULL;
-		if (ACPI_FAILURE(status))
-			return -EINVAL;
-		add_taint(TAINT_OVERRIDDEN_ACPI_TABLE);
-	}
-
-	return count;
-}
-
-static const struct file_operations cm_fops = {
-	.write = cm_write,
-	.llseek = default_llseek,
-};
-
-static int __init acpi_custom_method_init(void)
-{
-	if (!acpi_debugfs_dir)
-		return -ENOENT;
-
-	cm_dentry = debugfs_create_file("custom_method", S_IWUSR,
-					acpi_debugfs_dir, NULL, &cm_fops);
-	if (!cm_dentry)
-		return -ENODEV;
-
-	return 0;
-}
+EXPORT_SYMBOL_GPL(acpi_debugfs_dir);
 
 void __init acpi_debugfs_init(void)
 {
 	acpi_debugfs_dir = debugfs_create_dir("acpi", NULL);
-
-	acpi_custom_method_init();
 }