platform/x86: wmi: create userspace interface for drivers

For WMI operations that are only Set or Query readable and writable sysfs
attributes created by WMI vendor drivers or the bus driver makes sense.

For other WMI operations that are run on Method, there needs to be a
way to guarantee to userspace that the results from the method call
belong to the data request to the method call.  Sysfs attributes don't
work well in this scenario because two userspace processes may be
competing at reading/writing an attribute and step on each other's
data.

When a WMI vendor driver declares a callback method in the wmi_driver
the WMI bus driver will create a character device that maps to that
function.  This callback method will be responsible for filtering
invalid requests and performing the actual call.

That character device will correspond to this path:
/dev/wmi/$driver

Performing read() on this character device will provide the size
of the buffer that the character device needs to perform calls.
This buffer size can be set by vendor drivers through a new symbol
or when MOF parsing is available by the MOF.

Performing ioctl() on this character device will be interpretd
by the WMI bus driver. It will perform sanity tests for size of
data, test them for a valid instance, copy the data from userspace
and pass iton to the vendor driver to further process and run.

This creates an implicit policy that each driver will only be allowed
a single character device.  If a module matches multiple GUID's,
the wmi_devices will need to be all handled by the same wmi_driver.

The WMI vendor drivers will be responsible for managing inappropriate
access to this character device and proper locking on data used by
it.

When a WMI vendor driver is unloaded the WMI bus driver will clean
up the character device and any memory allocated for the call.

Signed-off-by: Mario Limonciello <mario.limonciello@dell.com>
Reviewed-by: Edward O'Callaghan <quasisec@google.com>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>

MAINTAINERS

@@ -384,6 +384,7 @@ ACPI WMI DRIVER
 L:	platform-driver-x86@vger.kernel.org
 S:	Orphan
 F:	drivers/platform/x86/wmi.c
+F:	include/uapi/linux/wmi.h
 
 AD1889 ALSA SOUND DRIVER
 M:	Thibaut Varene <T-Bone@parisc-linux.org>

drivers/platform/x86/wmi.c

@@ -38,12 +38,15 @@
 #include <linux/init.h>
 #include <linux/kernel.h>
 #include <linux/list.h>
+#include <linux/miscdevice.h>
 #include <linux/module.h>
 #include <linux/platform_device.h>
 #include <linux/slab.h>
 #include <linux/types.h>
+#include <linux/uaccess.h>
 #include <linux/uuid.h>
 #include <linux/wmi.h>
+#include <uapi/linux/wmi.h>
 
 ACPI_MODULE_NAME("wmi");
 MODULE_AUTHOR("Carlos Corbacho");
@@ -69,9 +72,12 @@ struct wmi_block {
 	struct wmi_device dev;
 	struct list_head list;
 	struct guid_block gblock;
+	struct miscdevice char_dev;
+	struct mutex char_mutex;
 	struct acpi_device *acpi_device;
 	wmi_notify_handler handler;
 	void *handler_data;
+	u64 req_buf_size;
 
 	bool read_takes_no_args;
 };
@@ -188,6 +194,25 @@ static acpi_status wmi_method_enable(struct wmi_block *wblock, int enable)
 /*
  * Exported WMI functions
  */
+
+/**
+ * set_required_buffer_size - Sets the buffer size needed for performing IOCTL
+ * @wdev: A wmi bus device from a driver
+ * @instance: Instance index
+ *
+ * Allocates memory needed for buffer, stores the buffer size in that memory
+ */
+int set_required_buffer_size(struct wmi_device *wdev, u64 length)
+{
+	struct wmi_block *wblock;
+
+	wblock = container_of(wdev, struct wmi_block, dev);
+	wblock->req_buf_size = length;
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(set_required_buffer_size);
+
 /**
  * wmi_evaluate_method - Evaluate a WMI method
  * @guid_string: 36 char string of the form fa50ff2b-f2e8-45de-83fa-65417f2f49ba
@@ -764,6 +789,111 @@ static int wmi_dev_match(struct device *dev, struct device_driver *driver)
 
 	return 0;
 }
+static int wmi_char_open(struct inode *inode, struct file *filp)
+{
+	const char *driver_name = filp->f_path.dentry->d_iname;
+	struct wmi_block *wblock = NULL;
+	struct wmi_block *next = NULL;
+
+	list_for_each_entry_safe(wblock, next, &wmi_block_list, list) {
+		if (!wblock->dev.dev.driver)
+			continue;
+		if (strcmp(driver_name, wblock->dev.dev.driver->name) == 0) {
+			filp->private_data = wblock;
+			break;
+		}
+	}
+
+	if (!filp->private_data)
+		return -ENODEV;
+
+	return nonseekable_open(inode, filp);
+}
+
+static ssize_t wmi_char_read(struct file *filp, char __user *buffer,
+	size_t length, loff_t *offset)
+{
+	struct wmi_block *wblock = filp->private_data;
+
+	return simple_read_from_buffer(buffer, length, offset,
+				       &wblock->req_buf_size,
+				       sizeof(wblock->req_buf_size));
+}
+
+static long wmi_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
+{
+	struct wmi_ioctl_buffer __user *input =
+		(struct wmi_ioctl_buffer __user *) arg;
+	struct wmi_block *wblock = filp->private_data;
+	struct wmi_ioctl_buffer *buf = NULL;
+	struct wmi_driver *wdriver = NULL;
+	int ret;
+
+	if (_IOC_TYPE(cmd) != WMI_IOC)
+		return -ENOTTY;
+
+	/* make sure we're not calling a higher instance than exists*/
+	if (_IOC_NR(cmd) >= wblock->gblock.instance_count)
+		return -EINVAL;
+
+	mutex_lock(&wblock->char_mutex);
+	buf = wblock->handler_data;
+	if (get_user(buf->length, &input->length)) {
+		dev_dbg(&wblock->dev.dev, "Read length from user failed\n");
+		ret = -EFAULT;
+		goto out_ioctl;
+	}
+	/* if it's too small, abort */
+	if (buf->length < wblock->req_buf_size) {
+		dev_err(&wblock->dev.dev,
+			"Buffer %lld too small, need at least %lld\n",
+			buf->length, wblock->req_buf_size);
+		ret = -EINVAL;
+		goto out_ioctl;
+	}
+	/* if it's too big, warn, driver will only use what is needed */
+	if (buf->length > wblock->req_buf_size)
+		dev_warn(&wblock->dev.dev,
+			"Buffer %lld is bigger than required %lld\n",
+			buf->length, wblock->req_buf_size);
+
+	/* copy the structure from userspace */
+	if (copy_from_user(buf, input, wblock->req_buf_size)) {
+		dev_dbg(&wblock->dev.dev, "Copy %llu from user failed\n",
+			wblock->req_buf_size);
+		ret = -EFAULT;
+		goto out_ioctl;
+	}
+
+	/* let the driver do any filtering and do the call */
+	wdriver = container_of(wblock->dev.dev.driver,
+			       struct wmi_driver, driver);
+	if (!try_module_get(wdriver->driver.owner))
+		return -EBUSY;
+	ret = wdriver->filter_callback(&wblock->dev, cmd, buf);
+	module_put(wdriver->driver.owner);
+	if (ret)
+		goto out_ioctl;
+
+	/* return the result (only up to our internal buffer size) */
+	if (copy_to_user(input, buf, wblock->req_buf_size)) {
+		dev_dbg(&wblock->dev.dev, "Copy %llu to user failed\n",
+			wblock->req_buf_size);
+		ret = -EFAULT;
+	}
+
+out_ioctl:
+	mutex_unlock(&wblock->char_mutex);
+	return ret;
+}
+
+static const struct file_operations wmi_fops = {
+	.owner		= THIS_MODULE,
+	.read		= wmi_char_read,
+	.open		= wmi_char_open,
+	.unlocked_ioctl	= wmi_ioctl,
+	.compat_ioctl	= wmi_ioctl,
+};
 
 static int wmi_dev_probe(struct device *dev)
 {
@@ -771,16 +901,63 @@ static int wmi_dev_probe(struct device *dev)
 	struct wmi_driver *wdriver =
 		container_of(dev->driver, struct wmi_driver, driver);
 	int ret = 0;
+	int count;
+	char *buf;
 
 	if (ACPI_FAILURE(wmi_method_enable(wblock, 1)))
 		dev_warn(dev, "failed to enable device -- probing anyway\n");
 
 	if (wdriver->probe) {
 		ret = wdriver->probe(dev_to_wdev(dev));
-		if (ret != 0 && ACPI_FAILURE(wmi_method_enable(wblock, 0)))
+		if (ret != 0)
-			dev_warn(dev, "failed to disable device\n");
+			goto probe_failure;
 	}
 
+	/* driver wants a character device made */
+	if (wdriver->filter_callback) {
+		/* check that required buffer size declared by driver or MOF */
+		if (!wblock->req_buf_size) {
+			dev_err(&wblock->dev.dev,
+				"Required buffer size not set\n");
+			ret = -EINVAL;
+			goto probe_failure;
+		}
+
+		count = get_order(wblock->req_buf_size);
+		wblock->handler_data = (void *)__get_free_pages(GFP_KERNEL,
+								count);
+		if (!wblock->handler_data) {
+			ret = -ENOMEM;
+			goto probe_failure;
+		}
+
+		buf = kmalloc(strlen(wdriver->driver.name) + 4, GFP_KERNEL);
+		if (!buf) {
+			ret = -ENOMEM;
+			goto probe_string_failure;
+		}
+		sprintf(buf, "wmi/%s", wdriver->driver.name);
+		wblock->char_dev.minor = MISC_DYNAMIC_MINOR;
+		wblock->char_dev.name = buf;
+		wblock->char_dev.fops = &wmi_fops;
+		wblock->char_dev.mode = 0444;
+		ret = misc_register(&wblock->char_dev);
+		if (ret) {
+			dev_warn(dev, "failed to register char dev: %d", ret);
+			ret = -ENOMEM;
+			goto probe_misc_failure;
+		}
+	}
+
+	return 0;
+
+probe_misc_failure:
+	kfree(buf);
+probe_string_failure:
+	kfree(wblock->handler_data);
+probe_failure:
+	if (ACPI_FAILURE(wmi_method_enable(wblock, 0)))
+		dev_warn(dev, "failed to disable device\n");
 	return ret;
 }
 
@@ -791,6 +968,13 @@ static int wmi_dev_remove(struct device *dev)
 		container_of(dev->driver, struct wmi_driver, driver);
 	int ret = 0;
 
+	if (wdriver->filter_callback) {
+		misc_deregister(&wblock->char_dev);
+		kfree(wblock->char_dev.name);
+		free_pages((unsigned long)wblock->handler_data,
+			   get_order(wblock->req_buf_size));
+	}
+
 	if (wdriver->remove)
 		ret = wdriver->remove(dev_to_wdev(dev));
 
@@ -847,6 +1031,7 @@ static int wmi_create_device(struct device *wmi_bus_dev,
 
 	if (gblock->flags & ACPI_WMI_METHOD) {
 		wblock->dev.dev.type = &wmi_type_method;
+		mutex_init(&wblock->char_mutex);
 		goto out_init;
 	}
 

include/linux/wmi.h

@@ -18,6 +18,7 @@
 
 #include <linux/device.h>
 #include <linux/acpi.h>
+#include <uapi/linux/wmi.h>
 
 struct wmi_device {
 	struct device dev;
@@ -36,6 +37,8 @@ extern acpi_status wmidev_evaluate_method(struct wmi_device *wdev,
 extern union acpi_object *wmidev_block_query(struct wmi_device *wdev,
 					     u8 instance);
 
+extern int set_required_buffer_size(struct wmi_device *wdev, u64 length);
+
 struct wmi_device_id {
 	const char *guid_string;
 };
@@ -47,6 +50,8 @@ struct wmi_driver {
 	int (*probe)(struct wmi_device *wdev);
 	int (*remove)(struct wmi_device *wdev);
 	void (*notify)(struct wmi_device *device, union acpi_object *data);
+	long (*filter_callback)(struct wmi_device *wdev, unsigned int cmd,
+				struct wmi_ioctl_buffer *arg);
 };
 
 extern int __must_check __wmi_driver_register(struct wmi_driver *driver,

include/uapi/linux/wmi.h

@@ -0,0 +1,26 @@
+/*
+ *  User API methods for ACPI-WMI mapping driver
+ *
+ *  Copyright (C) 2017 Dell, Inc.
+ *
+ *  This program is free software; you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License version 2 as
+ *  published by the Free Software Foundation.
+ */
+#ifndef _UAPI_LINUX_WMI_H
+#define _UAPI_LINUX_WMI_H
+
+#include <linux/types.h>
+
+/* WMI bus will filter all WMI vendor driver requests through this IOC */
+#define WMI_IOC 'W'
+
+/* All ioctl requests through WMI should declare their size followed by
+ * relevant data objects
+ */
+struct wmi_ioctl_buffer {
+	__u64	length;
+	__u8	data[];
+};
+
+#endif