USB: chaoskey: fix use-after-free on release

BugLink: https://bugs.launchpad.net/bugs/1848780

commit 93ddb1f5 upstream.

The driver was accessing its struct usb_interface in its release()
callback without holding a reference. This would lead to a
use-after-free whenever the device was disconnected while the character
device was still open.

Fixes: 66e3e591 ("usb: Add driver for Altus Metrum ChaosKey device (v2)")
Cc: stable <stable@vger.kernel.org>     # 4.1
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20191009153848.8664-3-johan@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

drivers/usb/misc/chaoskey.c

@@ -96,6 +96,7 @@ static void chaoskey_free(struct chaoskey *dev)
 	usb_dbg(dev->interface, "free");
 	kfree(dev->name);
 	kfree(dev->buf);
+	usb_put_intf(dev->interface);
 	kfree(dev);
 }
 
@@ -144,6 +145,8 @@ static int chaoskey_probe(struct usb_interface *interface,
 	if (dev == NULL)
 		return -ENOMEM;
 
+	dev->interface = usb_get_intf(interface);
+
 	dev->buf = kmalloc(size, GFP_KERNEL);
 
 	if (dev->buf == NULL) {
@@ -169,8 +172,6 @@ static int chaoskey_probe(struct usb_interface *interface,
 		strcat(dev->name, udev->serial);
 	}
 
-	dev->interface = interface;
-
 	dev->in_ep = in_ep;
 
 	dev->size = size;