[PATCH v4 2/3] Input: applespi - fix NULL pointer dereference in tp_dim open

[Shih-Yuan Lee <fourdollars@debian.org>]

The tp_dim debugfs file is registered synchronously during driver probe
in applespi_probe(). However, the applespi->touchpad_input_dev is
initialized and registered asynchronously in the driver's worker thread.

If a userspace process opens the debugfs file before the worker thread
has completed initialization, applespi_tp_dim_open() will dereference
the NULL applespi->touchpad_input_dev pointer, causing a kernel panic.

Fix this by using smp_load_acquire() to safely load touchpad_input_dev
and return -ENODEV if it is not yet initialized.

Signed-off-by: Shih-Yuan Lee <fourdollars@debian.org>
---
 drivers/input/keyboard/applespi.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/input/keyboard/applespi.c b/drivers/input/keyboard/applespi.c
index 3bdb9e7cfb8b..4d339445e9c7 100644
--- a/drivers/input/keyboard/applespi.c
+++ b/drivers/input/keyboard/applespi.c
@@ -963,12 +963,18 @@ static void applespi_debug_update_dimensions(struct applespi_data *applespi,
 static int applespi_tp_dim_open(struct inode *inode, struct file *file)
 {
 	struct applespi_data *applespi = inode->i_private;
+	struct input_dev *touchpad;
 
 	file->private_data = applespi;
 
+	/* Pairs with smp_store_release in applespi_register_touchpad_device() */
+	touchpad = smp_load_acquire(&applespi->touchpad_input_dev);
+	if (!touchpad)
+		return -ENODEV;
+
 	snprintf(applespi->tp_dim_val, sizeof(applespi->tp_dim_val),
 		 "0x%.4x %dx%d+%u+%u\n",
-		 applespi->touchpad_input_dev->id.product,
+		 touchpad->id.product,
 		 applespi->tp_dim_min_x, applespi->tp_dim_min_y,
 		 applespi->tp_dim_max_x - applespi->tp_dim_min_x,
 		 applespi->tp_dim_max_y - applespi->tp_dim_min_y);
-- 
2.39.5