ACPI: Limit access to custom_method

BugLink: http://bugs.launchpad.net/bugs/1566221

custom_method effectively allows arbitrary access to system memory, making
it possible for an attacker to circumvent restrictions on module loading.
Disable it if any such restrictions have been enabled.
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

drivers/acpi/custom_method.c

@@ -29,6 +29,9 @@ static ssize_t cm_write(struct file *file, const char __user * user_buf,
 	struct acpi_table_header table;
 	acpi_status status;
 
+	if (secure_modules())
+		return -EPERM;
+
 	if (!(*ppos)) {
 		/* parse the table header to get the table length */
 		if (count <= sizeof(struct acpi_table_header))