[PATCH 2/4] USB: legousbtower: fix deadlock on disconnect

[Johan Hovold <johan@kernel.org>]

Fix a potential deadlock if disconnect races with open.

Since commit d4ead16f50f9 ("USB: prevent char device open/deregister
race") core holds an rw-semaphore while open is called and when
releasing the minor number during deregistration. This can lead to an
ABBA deadlock if a driver takes a lock in open which it also holds
during deregistration.

This effectively reverts commit 78663ecc344b ("USB: disconnect open race
in legousbtower") which needlessly introduced this issue after a generic
fix for this race had been added to core by commit d4ead16f50f9 ("USB:
prevent char device open/deregister race").

Fixes: 78663ecc344b ("USB: disconnect open race in legousbtower")
Cc: stable <stable@vger.kernel.org>	# 2.6.24
Reported-by: syzbot+f9549f5ee8a5416f0b95@syzkaller.appspotmail.com
Tested-by: syzbot+f9549f5ee8a5416f0b95@syzkaller.appspotmail.com
Signed-off-by: Johan Hovold <johan@kernel.org>
---
 drivers/usb/misc/legousbtower.c | 19 ++-----------------
 1 file changed, 2 insertions(+), 17 deletions(-)

diff --git a/drivers/usb/misc/legousbtower.c b/drivers/usb/misc/legousbtower.c
index 1db07d4dc738..773e4188f336 100644
--- a/drivers/usb/misc/legousbtower.c
+++ b/drivers/usb/misc/legousbtower.c
@@ -179,7 +179,6 @@ static const struct usb_device_id tower_table[] = {
 };
 
 MODULE_DEVICE_TABLE (usb, tower_table);
-static DEFINE_MUTEX(open_disc_mutex);
 
 #define LEGO_USB_TOWER_MINOR_BASE	160
 
@@ -332,18 +331,14 @@ static int tower_open (struct inode *inode, struct file *file)
 		goto exit;
 	}
 
-	mutex_lock(&open_disc_mutex);
 	dev = usb_get_intfdata(interface);
-
 	if (!dev) {
-		mutex_unlock(&open_disc_mutex);
 		retval = -ENODEV;
 		goto exit;
 	}
 
 	/* lock this device */
 	if (mutex_lock_interruptible(&dev->lock)) {
-		mutex_unlock(&open_disc_mutex);
 	        retval = -ERESTARTSYS;
 		goto exit;
 	}
@@ -351,12 +346,10 @@ static int tower_open (struct inode *inode, struct file *file)
 
 	/* allow opening only once */
 	if (dev->open_count) {
-		mutex_unlock(&open_disc_mutex);
 		retval = -EBUSY;
 		goto unlock_exit;
 	}
 	dev->open_count = 1;
-	mutex_unlock(&open_disc_mutex);
 
 	/* reset the tower */
 	result = usb_control_msg (dev->udev,
@@ -423,10 +416,9 @@ static int tower_release (struct inode *inode, struct file *file)
 
 	if (dev == NULL) {
 		retval = -ENODEV;
-		goto exit_nolock;
+		goto exit;
 	}
 
-	mutex_lock(&open_disc_mutex);
 	if (mutex_lock_interruptible(&dev->lock)) {
 	        retval = -ERESTARTSYS;
 		goto exit;
@@ -456,10 +448,7 @@ static int tower_release (struct inode *inode, struct file *file)
 
 unlock_exit:
 	mutex_unlock(&dev->lock);
-
 exit:
-	mutex_unlock(&open_disc_mutex);
-exit_nolock:
 	return retval;
 }
 
@@ -912,7 +901,6 @@ static int tower_probe (struct usb_interface *interface, const struct usb_device
 	if (retval) {
 		/* something prevented us from registering this driver */
 		dev_err(idev, "Not able to get a minor for this device.\n");
-		usb_set_intfdata (interface, NULL);
 		goto error;
 	}
 	dev->minor = interface->minor;
@@ -944,16 +932,13 @@ static void tower_disconnect (struct usb_interface *interface)
 	int minor;
 
 	dev = usb_get_intfdata (interface);
-	mutex_lock(&open_disc_mutex);
-	usb_set_intfdata (interface, NULL);
 
 	minor = dev->minor;
 
-	/* give back our minor */
+	/* give back our minor and prevent further open() */
 	usb_deregister_dev (interface, &tower_class);
 
 	mutex_lock(&dev->lock);
-	mutex_unlock(&open_disc_mutex);
 
 	/* if the device is not opened, then we clean up right now */
 	if (!dev->open_count) {
-- 
2.23.0