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Subject[PATCH 3.16 025/136] USB: legousbtower: fix deadlock on disconnect
3.16.80-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Johan Hovold <johan@kernel.org>

commit 33a7813219f208f4952ece60ee255fd983272dec upstream.

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")
Reported-by: syzbot+f9549f5ee8a5416f0b95@syzkaller.appspotmail.com
Tested-by: syzbot+f9549f5ee8a5416f0b95@syzkaller.appspotmail.com
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20190919083039.30898-3-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
drivers/usb/misc/legousbtower.c | 19 ++-----------------
1 file changed, 2 insertions(+), 17 deletions(-)

--- a/drivers/usb/misc/legousbtower.c
+++ b/drivers/usb/misc/legousbtower.c
@@ -185,7 +185,6 @@ static const struct usb_device_id tower_
};

MODULE_DEVICE_TABLE (usb, tower_table);
-static DEFINE_MUTEX(open_disc_mutex);

#define LEGO_USB_TOWER_MINOR_BASE 160

@@ -338,18 +337,14 @@ static int tower_open (struct inode *ino
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;
}
@@ -357,12 +352,10 @@ static int tower_open (struct inode *ino

/* 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,
@@ -429,10 +422,9 @@ static int tower_release (struct inode *

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;
@@ -462,10 +454,7 @@ static int tower_release (struct inode *

unlock_exit:
mutex_unlock(&dev->lock);
-
exit:
- mutex_unlock(&open_disc_mutex);
-exit_nolock:
return retval;
}

@@ -944,7 +933,6 @@ static int tower_probe (struct usb_inter
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;
@@ -976,16 +964,13 @@ static void tower_disconnect (struct usb
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) {
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