[PATCH] gpiolib: fix invalid pointer access in debugfs

[Bartosz Golaszewski <brgl@bgdev.pl>]

From: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

If the memory allocation in gpiolib_seq_start() fails, the s->private
field remains uninitialized and is later dereferenced without checking
in gpiolib_seq_stop(). Initialize s->private to NULL before calling
kzalloc() and check it before dereferencing it.

Fixes: e348544f7994 ("gpio: protect the list of GPIO devices with SRCU")
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
---
This was brought to my attention by a person under sanctions. This is a
simplified version of their initial patch.

 drivers/gpio/gpiolib.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
index a81981336b36..fdb6a002dbda 100644
--- a/drivers/gpio/gpiolib.c
+++ b/drivers/gpio/gpiolib.c
@@ -5303,6 +5303,8 @@ static void *gpiolib_seq_start(struct seq_file *s, loff_t *pos)
 	struct gpio_device *gdev;
 	loff_t index = *pos;
 
+	s->private = NULL;
+
 	priv = kzalloc(sizeof(*priv), GFP_KERNEL);
 	if (!priv)
 		return NULL;
@@ -5338,6 +5340,9 @@ static void gpiolib_seq_stop(struct seq_file *s, void *v)
 {
 	struct gpiolib_seq_priv *priv = s->private;
 
+	if (!priv)
+		return;
+
 	srcu_read_unlock(&gpio_devices_srcu, priv->idx);
 	kfree(priv);
 }
-- 
2.48.1