[PATCH v2] carl9170: fix misuse of device driver API

[Christian Lamparter <chunkeey@gmail.com>]

This patch follows Alan Stern's recent patch:
"p54: Fix race between disconnect and firmware loading"

that overhauled carl9170 buggy firmware loading and driver
unbinding procedures.

Since the carl9170 code was adapted from p54 it uses the
same functions and is likely to have the same problem, but
it's just that the syzbot hasn't reproduce them (yet).

a summary from the changes (copied from the p54 patch):
 * Call usb_driver_release_interface() rather than
   device_release_driver().

 * Lock udev (the interface's parent) before unbinding the
   driver instead of locking udev->parent.

 * During the firmware loading process, take a reference
   to the USB interface instead of the USB device.

 * Don't take an unnecessary reference to the device during
   probe (and then don't drop it during disconnect).

and

 * Make sure to prevent use-after-free bugs by explicitly
   setting the driver context to NULL after signaling the
   completion.

Cc: <stable@vger.kernel.org>
Cc: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
---
v2: Alan Stern's comments
  - fixed possible use-after-free
---
 drivers/net/wireless/ath/carl9170/usb.c | 39 +++++++++++--------------
 1 file changed, 17 insertions(+), 22 deletions(-)

diff --git a/drivers/net/wireless/ath/carl9170/usb.c b/drivers/net/wireless/ath/carl9170/usb.c
index e7c3f3b8457d..99f1897a775d 100644
--- a/drivers/net/wireless/ath/carl9170/usb.c
+++ b/drivers/net/wireless/ath/carl9170/usb.c
@@ -128,6 +128,8 @@ static const struct usb_device_id carl9170_usb_ids[] = {
 };
 MODULE_DEVICE_TABLE(usb, carl9170_usb_ids);
 
+static struct usb_driver carl9170_driver;
+
 static void carl9170_usb_submit_data_urb(struct ar9170 *ar)
 {
 	struct urb *urb;
@@ -966,32 +968,28 @@ static int carl9170_usb_init_device(struct ar9170 *ar)
 
 static void carl9170_usb_firmware_failed(struct ar9170 *ar)
 {
-	struct device *parent = ar->udev->dev.parent;
-	struct usb_device *udev;
-
-	/*
-	 * Store a copy of the usb_device pointer locally.
-	 * This is because device_release_driver initiates
-	 * carl9170_usb_disconnect, which in turn frees our
-	 * driver context (ar).
+	/* Store a copies of the usb_interface and usb_device pointer locally.
+	 * This is because release_driver initiates carl9170_usb_disconnect,
+	 * which in turn frees our driver context (ar).
 	 */
-	udev = ar->udev;
+	struct usb_interface *intf = ar->intf;
+	struct usb_device *udev = ar->udev;
 
 	complete(&ar->fw_load_wait);
+	/* at this point 'ar' could be already freed. Don't use it anymore */
+	ar = NULL;
 
 	/* unbind anything failed */
-	if (parent)
-		device_lock(parent);
-
-	device_release_driver(&udev->dev);
-	if (parent)
-		device_unlock(parent);
+	usb_lock_device(udev);
+	usb_driver_release_interface(&carl9170_driver, intf);
+	usb_unlock_device(udev);
 
-	usb_put_dev(udev);
+	usb_put_intf(intf);
 }
 
 static void carl9170_usb_firmware_finish(struct ar9170 *ar)
 {
+	struct usb_interface *intf = ar->intf;
 	int err;
 
 	err = carl9170_parse_firmware(ar);
@@ -1009,7 +1007,7 @@ static void carl9170_usb_firmware_finish(struct ar9170 *ar)
 		goto err_unrx;
 
 	complete(&ar->fw_load_wait);
-	usb_put_dev(ar->udev);
+	usb_put_intf(intf);
 	return;
 
 err_unrx:
@@ -1052,7 +1050,6 @@ static int carl9170_usb_probe(struct usb_interface *intf,
 		return PTR_ERR(ar);
 
 	udev = interface_to_usbdev(intf);
-	usb_get_dev(udev);
 	ar->udev = udev;
 	ar->intf = intf;
 	ar->features = id->driver_info;
@@ -1094,15 +1091,14 @@ static int carl9170_usb_probe(struct usb_interface *intf,
 	atomic_set(&ar->rx_anch_urbs, 0);
 	atomic_set(&ar->rx_pool_urbs, 0);
 
-	usb_get_dev(ar->udev);
+	usb_get_intf(intf);
 
 	carl9170_set_state(ar, CARL9170_STOPPED);
 
 	err = request_firmware_nowait(THIS_MODULE, 1, CARL9170FW_NAME,
 		&ar->udev->dev, GFP_KERNEL, ar, carl9170_usb_firmware_step2);
 	if (err) {
-		usb_put_dev(udev);
-		usb_put_dev(udev);
+		usb_put_intf(intf);
 		carl9170_free(ar);
 	}
 	return err;
@@ -1131,7 +1127,6 @@ static void carl9170_usb_disconnect(struct usb_interface *intf)
 
 	carl9170_release_firmware(ar);
 	carl9170_free(ar);
-	usb_put_dev(udev);
 }
 
 #ifdef CONFIG_PM
-- 
2.20.1