[PATCH v4 1/2] hwmon: (powerz) Fix use-after-free on USB disconnect

[Pradhan, Sanman]

From: Sanman Pradhan <psanman@xxxxxxxxxxx>

After powerz_disconnect() frees the URB and releases the mutex, a
subsequent powerz_read() call can acquire the mutex and call
powerz_read_data(), which dereferences the freed URB pointer.

Fix by:
 - Setting priv->urb to NULL in powerz_disconnect() so that
   powerz_read_data() can detect the disconnected state.
 - Adding a !priv->urb check at the start of powerz_read_data()
   to return -ENODEV on a disconnected device.
 - Moving usb_set_intfdata() before hwmon registration so the
   disconnect handler can always find the priv pointer.

Fixes: 4381a36abdf1c ("hwmon: add POWER-Z driver")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Sanman Pradhan <psanman@xxxxxxxxxxx>
---
v4:
 - Split from combined patch into standalone disconnect fix
 - Reword commit message per review feedback
 - Drop usb_set_intfdata(intf, NULL) in probe error path
   and disconnect (unnecessary)
v1-v3:
 - Part of combined patch "Fix use-after-free and signal
   handling on USB disconnect"

 drivers/hwmon/powerz.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/hwmon/powerz.c b/drivers/hwmon/powerz.c
index 4e663d5b4e33..a75b941bd6e2 100644
--- a/drivers/hwmon/powerz.c
+++ b/drivers/hwmon/powerz.c
@@ -108,6 +108,9 @@ static int powerz_read_data(struct usb_device *udev, struct powerz_priv *priv)
 {
 	int ret;
 
+	if (!priv->urb)
+		return -ENODEV;
+
 	priv->status = -ETIMEDOUT;
 	reinit_completion(&priv->completion);
 
@@ -224,6 +227,8 @@ static int powerz_probe(struct usb_interface *intf,
 	mutex_init(&priv->mutex);
 	init_completion(&priv->completion);
 
+	usb_set_intfdata(intf, priv);
+
 	hwmon_dev =
 	    devm_hwmon_device_register_with_info(parent, DRIVER_NAME, priv,
 						 &powerz_chip_info, NULL);
@@ -232,8 +237,6 @@ static int powerz_probe(struct usb_interface *intf,
 		return PTR_ERR(hwmon_dev);
 	}
 
-	usb_set_intfdata(intf, priv);
-
 	return 0;
 }
 
@@ -244,6 +247,7 @@ static void powerz_disconnect(struct usb_interface *intf)
 	mutex_lock(&priv->mutex);
 	usb_kill_urb(priv->urb);
 	usb_free_urb(priv->urb);
+	priv->urb = NULL;
 	mutex_unlock(&priv->mutex);
 }
 
-- 
2.34.1