[v7,2/2] gpiolib: protect the GPIO device against being dropped while in use by user-space

diff --git a/drivers/gpio/gpiolib-cdev.c b/drivers/gpio/gpiolib-cdev.c
index 6fa5c2169985..c8138ae382e8 100644
--- a/drivers/gpio/gpiolib-cdev.c
+++ b/drivers/gpio/gpiolib-cdev.c
@@ -55,6 +55,53 @@  static_assert(IS_ALIGNED(sizeof(struct gpio_v2_line_values), 8));
  * interface to gpiolib GPIOs via ioctl()s.
  */
 
+typedef __poll_t (*poll_fn)(struct file *, struct poll_table_struct *);
+typedef long (*ioctl_fn)(struct file *, unsigned int, unsigned long);
+typedef ssize_t (*read_fn)(struct file *, char __user *,
+			   size_t count, loff_t *);
+
+static __poll_t call_poll_locked(struct file *file,
+				 struct poll_table_struct *wait,
+				 struct gpio_device *gdev, poll_fn func)
+{
+	__poll_t ret;
+
+	if (!down_read_trylock(&gdev->sem))
+		return EPOLLHUP | EPOLLERR;
+	ret = func(file, wait);
+	up_read(&gdev->sem);
+
+	return ret;
+}
+
+static long call_ioctl_locked(struct file *file, unsigned int cmd,
+			      unsigned long arg, struct gpio_device *gdev,
+			      ioctl_fn func)
+{
+	long ret;
+
+	if (!down_read_trylock(&gdev->sem))
+		return -ENODEV;
+	ret = func(file, cmd, arg);
+	up_read(&gdev->sem);
+
+	return ret;
+}
+
+static ssize_t call_read_locked(struct file *file, char __user *buf,
+				size_t count, loff_t *f_ps,
+				struct gpio_device *gdev, read_fn func)
+{
+	ssize_t ret;
+
+	if (!down_read_trylock(&gdev->sem))
+		return -ENODEV;
+	ret = func(file, buf, count, f_ps);
+	up_read(&gdev->sem);
+
+	return ret;
+}
+
 /*
  * GPIO line handle management
  */
@@ -191,8 +238,8 @@  static long linehandle_set_config(struct linehandle_state *lh,
 	return 0;
 }
 
-static long linehandle_ioctl(struct file *file, unsigned int cmd,
-			     unsigned long arg)
+static long linehandle_ioctl_unlocked(struct file *file, unsigned int cmd,
+				      unsigned long arg)
 {
 	struct linehandle_state *lh = file->private_data;
 	void __user *ip = (void __user *)arg;
@@ -250,6 +297,15 @@  static long linehandle_ioctl(struct file *file, unsigned int cmd,
 	}
 }
 
+static long linehandle_ioctl(struct file *file, unsigned int cmd,
+			     unsigned long arg)
+{
+	struct linehandle_state *lh = file->private_data;
+
+	return call_ioctl_locked(file, cmd, arg, lh->gdev,
+				 linehandle_ioctl_unlocked);
+}
+
 #ifdef CONFIG_COMPAT
 static long linehandle_ioctl_compat(struct file *file, unsigned int cmd,
 				    unsigned long arg)
@@ -1381,8 +1437,8 @@  static long linereq_set_config(struct linereq *lr, void __user *ip)
 	return ret;
 }
 
-static long linereq_ioctl(struct file *file, unsigned int cmd,
-			  unsigned long arg)
+static long linereq_ioctl_unlocked(struct file *file, unsigned int cmd,
+				   unsigned long arg)
 {
 	struct linereq *lr = file->private_data;
 	void __user *ip = (void __user *)arg;
@@ -1402,6 +1458,15 @@  static long linereq_ioctl(struct file *file, unsigned int cmd,
 	}
 }
 
+static long linereq_ioctl(struct file *file, unsigned int cmd,
+			  unsigned long arg)
+{
+	struct linereq *lr = file->private_data;
+
+	return call_ioctl_locked(file, cmd, arg, lr->gdev,
+				 linereq_ioctl_unlocked);
+}
+
 #ifdef CONFIG_COMPAT
 static long linereq_ioctl_compat(struct file *file, unsigned int cmd,
 				 unsigned long arg)
@@ -1410,8 +1475,8 @@  static long linereq_ioctl_compat(struct file *file, unsigned int cmd,
 }
 #endif
 
-static __poll_t linereq_poll(struct file *file,
-			    struct poll_table_struct *wait)
+static __poll_t linereq_poll_unlocked(struct file *file,
+				      struct poll_table_struct *wait)
 {
 	struct linereq *lr = file->private_data;
 	__poll_t events = 0;
@@ -1428,10 +1493,16 @@  static __poll_t linereq_poll(struct file *file,
 	return events;
 }
 
-static ssize_t linereq_read(struct file *file,
-			    char __user *buf,
-			    size_t count,
-			    loff_t *f_ps)
+static __poll_t linereq_poll(struct file *file,
+			     struct poll_table_struct *wait)
+{
+	struct linereq *lr = file->private_data;
+
+	return call_poll_locked(file, wait, lr->gdev, linereq_poll_unlocked);
+}
+
+static ssize_t linereq_read_unlocked(struct file *file, char __user *buf,
+				     size_t count, loff_t *f_ps)
 {
 	struct linereq *lr = file->private_data;
 	struct gpio_v2_line_event le;
@@ -1485,6 +1556,15 @@  static ssize_t linereq_read(struct file *file,
 	return bytes_read;
 }
 
+static ssize_t linereq_read(struct file *file, char __user *buf,
+			    size_t count, loff_t *f_ps)
+{
+	struct linereq *lr = file->private_data;
+
+	return call_read_locked(file, buf, count, f_ps, lr->gdev,
+				linereq_read_unlocked);
+}
+
 static void linereq_free(struct linereq *lr)
 {
 	unsigned int i;
@@ -1722,8 +1802,8 @@  struct lineevent_state {
 	(GPIOEVENT_REQUEST_RISING_EDGE | \
 	GPIOEVENT_REQUEST_FALLING_EDGE)
 
-static __poll_t lineevent_poll(struct file *file,
-			       struct poll_table_struct *wait)
+static __poll_t lineevent_poll_unlocked(struct file *file,
+					struct poll_table_struct *wait)
 {
 	struct lineevent_state *le = file->private_data;
 	__poll_t events = 0;
@@ -1739,15 +1819,21 @@  static __poll_t lineevent_poll(struct file *file,
 	return events;
 }
 
+static __poll_t lineevent_poll(struct file *file,
+			       struct poll_table_struct *wait)
+{
+	struct lineevent_state *le = file->private_data;
+
+	return call_poll_locked(file, wait, le->gdev, lineevent_poll_unlocked);
+}
+
 struct compat_gpioeevent_data {
 	compat_u64	timestamp;
 	u32		id;
 };
 
-static ssize_t lineevent_read(struct file *file,
-			      char __user *buf,
-			      size_t count,
-			      loff_t *f_ps)
+static ssize_t lineevent_read_unlocked(struct file *file, char __user *buf,
+				       size_t count, loff_t *f_ps)
 {
 	struct lineevent_state *le = file->private_data;
 	struct gpioevent_data ge;
@@ -1815,6 +1901,15 @@  static ssize_t lineevent_read(struct file *file,
 	return bytes_read;
 }
 
+static ssize_t lineevent_read(struct file *file, char __user *buf,
+			      size_t count, loff_t *f_ps)
+{
+	struct lineevent_state *le = file->private_data;
+
+	return call_read_locked(file, buf, count, f_ps, le->gdev,
+				lineevent_read_unlocked);
+}
+
 static void lineevent_free(struct lineevent_state *le)
 {
 	if (le->irq)
@@ -1832,8 +1927,8 @@  static int lineevent_release(struct inode *inode, struct file *file)
 	return 0;
 }
 
-static long lineevent_ioctl(struct file *file, unsigned int cmd,
-			    unsigned long arg)
+static long lineevent_ioctl_unlocked(struct file *file, unsigned int cmd,
+				     unsigned long arg)
 {
 	struct lineevent_state *le = file->private_data;
 	void __user *ip = (void __user *)arg;
@@ -1864,6 +1959,15 @@  static long lineevent_ioctl(struct file *file, unsigned int cmd,
 	return -EINVAL;
 }
 
+static long lineevent_ioctl(struct file *file, unsigned int cmd,
+			    unsigned long arg)
+{
+	struct lineevent_state *le = file->private_data;
+
+	return call_ioctl_locked(file, cmd, arg, le->gdev,
+				 lineevent_ioctl_unlocked);
+}
+
 #ifdef CONFIG_COMPAT
 static long lineevent_ioctl_compat(struct file *file, unsigned int cmd,
 				   unsigned long arg)
@@ -2422,8 +2526,8 @@  static int lineinfo_changed_notify(struct notifier_block *nb,
 	return NOTIFY_OK;
 }
 
-static __poll_t lineinfo_watch_poll(struct file *file,
-				    struct poll_table_struct *pollt)
+static __poll_t lineinfo_watch_poll_unlocked(struct file *file,
+					     struct poll_table_struct *pollt)
 {
 	struct gpio_chardev_data *cdev = file->private_data;
 	__poll_t events = 0;
@@ -2440,8 +2544,17 @@  static __poll_t lineinfo_watch_poll(struct file *file,
 	return events;
 }
 
-static ssize_t lineinfo_watch_read(struct file *file, char __user *buf,
-				   size_t count, loff_t *off)
+static __poll_t lineinfo_watch_poll(struct file *file,
+				    struct poll_table_struct *pollt)
+{
+	struct gpio_chardev_data *cdev = file->private_data;
+
+	return call_poll_locked(file, pollt, cdev->gdev,
+				lineinfo_watch_poll_unlocked);
+}
+
+static ssize_t lineinfo_watch_read_unlocked(struct file *file, char __user *buf,
+					    size_t count, loff_t *off)
 {
 	struct gpio_chardev_data *cdev = file->private_data;
 	struct gpio_v2_line_info_changed event;
@@ -2519,6 +2632,15 @@  static ssize_t lineinfo_watch_read(struct file *file, char __user *buf,
 	return bytes_read;
 }
 
+static ssize_t lineinfo_watch_read(struct file *file, char __user *buf,
+				   size_t count, loff_t *off)
+{
+	struct gpio_chardev_data *cdev = file->private_data;
+
+	return call_read_locked(file, buf, count, off, cdev->gdev,
+				lineinfo_watch_read_unlocked);
+}
+
 /**
  * gpio_chrdev_open() - open the chardev for ioctl operations
  * @inode: inode for this chardev
@@ -2532,13 +2654,17 @@  static int gpio_chrdev_open(struct inode *inode, struct file *file)
 	struct gpio_chardev_data *cdev;
 	int ret = -ENOMEM;
 
+	down_read(&gdev->sem);
+
 	/* Fail on open if the backing gpiochip is gone */
-	if (!gdev->chip)
-		return -ENODEV;
+	if (!gdev->chip) {
+		ret = -ENODEV;
+		goto out_unlock;
+	}
 
 	cdev = kzalloc(sizeof(*cdev), GFP_KERNEL);
 	if (!cdev)
-		return -ENOMEM;
+		goto out_unlock;
 
 	cdev->watched_lines = bitmap_zalloc(gdev->chip->ngpio, GFP_KERNEL);
 	if (!cdev->watched_lines)
@@ -2561,6 +2687,8 @@  static int gpio_chrdev_open(struct inode *inode, struct file *file)
 	if (ret)
 		goto out_unregister_notifier;
 
+	up_read(&gdev->sem);
+
 	return ret;
 
 out_unregister_notifier:
@@ -2570,6 +2698,8 @@  static int gpio_chrdev_open(struct inode *inode, struct file *file)
 	bitmap_free(cdev->watched_lines);
 out_free_cdev:
 	kfree(cdev);
+out_unlock:
+	up_read(&gdev->sem);
 	return ret;
 }
 
diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
index a70522aef355..5974cfc61b41 100644
--- a/drivers/gpio/gpiolib.c
+++ b/drivers/gpio/gpiolib.c
@@ -735,6 +735,7 @@  int gpiochip_add_data_with_key(struct gpio_chip *gc, void *data,
 	spin_unlock_irqrestore(&gpio_lock, flags);
 
 	BLOCKING_INIT_NOTIFIER_HEAD(&gdev->notifier);
+	init_rwsem(&gdev->sem);
 
 #ifdef CONFIG_PINCTRL
 	INIT_LIST_HEAD(&gdev->pin_ranges);
@@ -875,6 +876,8 @@  void gpiochip_remove(struct gpio_chip *gc)
 	unsigned long	flags;
 	unsigned int	i;
 
+	down_write(&gdev->sem);
+
 	/* FIXME: should the legacy sysfs handling be moved to gpio_device? */
 	gpiochip_sysfs_unregister(gdev);
 	gpiochip_free_hogs(gc);
@@ -909,6 +912,7 @@  void gpiochip_remove(struct gpio_chip *gc)
 	 * gone.
 	 */
 	gcdev_unregister(gdev);
+	up_write(&gdev->sem);
 	put_device(&gdev->dev);
 }
 EXPORT_SYMBOL_GPL(gpiochip_remove);
diff --git a/drivers/gpio/gpiolib.h b/drivers/gpio/gpiolib.h
index d900ecdbac46..9ad68a0adf4a 100644
--- a/drivers/gpio/gpiolib.h
+++ b/drivers/gpio/gpiolib.h
@@ -15,6 +15,7 @@ 
 #include <linux/device.h>
 #include <linux/module.h>
 #include <linux/cdev.h>
+#include <linux/rwsem.h>
 
 #define GPIOCHIP_NAME	"gpiochip"
 
@@ -39,6 +40,9 @@ 
  * @list: links gpio_device:s together for traversal
  * @notifier: used to notify subscribers about lines being requested, released
  *            or reconfigured
+ * @sem: protects the structure from a NULL-pointer dereference of @chip by
+ *       user-space operations when the device gets unregistered during
+ *       a hot-unplug event
  * @pin_ranges: range of pins served by the GPIO driver
  *
  * This state container holds most of the runtime variable data
@@ -60,6 +64,7 @@  struct gpio_device {
 	void			*data;
 	struct list_head        list;
 	struct blocking_notifier_head notifier;
+	struct rw_semaphore	sem;
 
 #ifdef CONFIG_PINCTRL
 	/*