gpio: sysfs: fix memory leaks and device hotplug

commit 483d8211 upstream.

Unregister GPIOs requested through sysfs at chip remove to avoid leaking
the associated memory and sysfs entries.

The stale sysfs entries prevented the gpio numbers from being exported
when the gpio range was later reused (e.g. at device reconnect).

This also fixes the related module-reference leak.

Note that kernfs makes sure that any on-going sysfs operations finish
before the class devices are unregistered and that further accesses
fail.

The chip exported flag is used to prevent gpiod exports during removal.
This also makes it harder to trigger, but does not fix, the related race
between gpiochip_remove and export_store, which is really a race with
gpiod_request that needs to be addressed separately.

Also note that this would prevent the crashes (e.g. NULL-dereferences)
at reconnect that affects pre-3.18 kernels, as well as use-after-free on
operations on open attribute files on pre-3.14 kernels (prior to
kernfs).

Fixes: d8f388d8 ("gpio: sysfs interface")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
[bwh: Backported to 3.2:
 - Adjust filename, context
 - Move up initialisation of 'desc' in gpio_export()
 - Use global 'gpio_desc' array and gpio_free() function in
   gpiochip_unexport()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/gpio/gpiolib.c

@@ -726,6 +726,7 @@ static struct class gpio_class = {
  */
 int gpio_export(unsigned gpio, bool direction_may_change)
 {
+	struct gpio_chip	*chip;
 	unsigned long		flags;
 	struct gpio_desc	*desc;
 	int			status;
@@ -743,10 +744,18 @@ int gpio_export(unsigned gpio, bool direction_may_change)
 		return -EINVAL;
 	}
 
+	desc = &gpio_desc[gpio];
+	chip = desc->chip;
+
 	mutex_lock(&sysfs_lock);
 
+	/* check if chip is being removed */
+	if (!chip || !chip->exported) {
+		status = -ENODEV;
+		goto fail_unlock;
+	}
+
 	spin_lock_irqsave(&gpio_lock, flags);
-	desc = &gpio_desc[gpio];
 	if (!test_bit(FLAG_REQUESTED, &desc->flags) ||
 	     test_bit(FLAG_EXPORT, &desc->flags)) {
 		spin_unlock_irqrestore(&gpio_lock, flags);
@@ -973,12 +982,15 @@ static void gpiochip_unexport(struct gpio_chip *chip)
 {
 	int			status;
 	struct device		*dev;
+	struct gpio_desc *desc;
+	unsigned int i;
 
 	mutex_lock(&sysfs_lock);
 	dev = class_find_device(&gpio_class, NULL, chip, match_export);
 	if (dev) {
 		put_device(dev);
 		device_unregister(dev);
+		/* prevent further gpiod exports */
 		chip->exported = 0;
 		status = 0;
 	} else
@@ -988,6 +1000,13 @@ static void gpiochip_unexport(struct gpio_chip *chip)
 	if (status)
 		pr_debug("%s: chip %s status %d\n", __func__,
 				chip->label, status);
+
+	/* unregister gpio class devices owned by sysfs */
+	for (i = 0; i < chip->ngpio; i++) {
+		desc = &gpio_desc[chip->base + i];
+		if (test_and_clear_bit(FLAG_SYSFS, &desc->flags))
+			gpio_free(chip->base + i);
+	}
 }
 
 static int __init gpiolib_sysfs_init(void)