USB: usb-skeleton: fix NULL-deref on disconnect

The driver was using its struct usb_interface pointer as an inverted
disconnected flag and was setting it to NULL before making sure all
completion handlers had run. This could lead to NULL-pointer
dereferences in the dev_err() statements in the completion handlers
which relies on said pointer.

Fix this by using a dedicated disconnected flag.

Note that this is also addresses a NULL-pointer dereference at release()
and a struct usb_interface reference leak introduced by a recent runtime
PM fix, which depends on and should have been submitted together with
this patch.

Fixes: 4212cd74 ("USB: usb-skeleton.c: remove err() usage")
Fixes: 5c290a5e ("USB: usb-skeleton: fix runtime PM after driver unbind")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20191009170944.30057-2-johan@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/usb-skeleton.c

@@ -61,6 +61,7 @@ struct usb_skel {
 	spinlock_t		err_lock;		/* lock for errors */
 	struct kref		kref;
 	struct mutex		io_mutex;		/* synchronize I/O with disconnect */
+	unsigned long		disconnected:1;
 	wait_queue_head_t	bulk_in_wait;		/* to wait for an ongoing read */
 };
 #define to_skel_dev(d) container_of(d, struct usb_skel, kref)
@@ -238,7 +239,7 @@ static ssize_t skel_read(struct file *file, char *buffer, size_t count,
 	if (rv < 0)
 		return rv;
 
-	if (!dev->interface) {		/* disconnect() was called */
+	if (dev->disconnected) {		/* disconnect() was called */
 		rv = -ENODEV;
 		goto exit;
 	}
@@ -420,7 +421,7 @@ static ssize_t skel_write(struct file *file, const char *user_buffer,
 
 	/* this lock makes sure we don't submit URBs to gone devices */
 	mutex_lock(&dev->io_mutex);
-	if (!dev->interface) {		/* disconnect() was called */
+	if (dev->disconnected) {		/* disconnect() was called */
 		mutex_unlock(&dev->io_mutex);
 		retval = -ENODEV;
 		goto error;
@@ -571,7 +572,7 @@ static void skel_disconnect(struct usb_interface *interface)
 
 	/* prevent more I/O from starting */
 	mutex_lock(&dev->io_mutex);
-	dev->interface = NULL;
+	dev->disconnected = 1;
 	mutex_unlock(&dev->io_mutex);
 
 	usb_kill_anchored_urbs(&dev->submitted);