xhci: Fix NULL pointer dereference in xhci_clear_tt_buffer_complete()

udev stored in ep->hcpriv might be NULL if tt buffer is cleared
due to a halted control endpoint during device enumeration

xhci_clear_tt_buffer_complete is called by hub_tt_work() once it's
scheduled,  and by then usb core might have freed and allocated a
new udev for the next enumeration attempt.

Fixes: ef513be0 ("usb: xhci: Add Clear_TT_Buffer")
Cc: <stable@vger.kernel.org> # v5.3
Reported-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://lore.kernel.org/r/1570190373-30684-9-git-send-email-mathias.nyman@linux.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/xhci.c

@@ -5238,8 +5238,16 @@ static void xhci_clear_tt_buffer_complete(struct usb_hcd *hcd,
 	unsigned int ep_index;
 	unsigned long flags;
 
+	/*
+	 * udev might be NULL if tt buffer is cleared during a failed device
+	 * enumeration due to a halted control endpoint. Usb core might
+	 * have allocated a new udev for the next enumeration attempt.
+	 */
+
 	xhci = hcd_to_xhci(hcd);
 	udev = (struct usb_device *)ep->hcpriv;
+	if (!udev)
+		return;
 	slot_id = udev->slot_id;
 	ep_index = xhci_get_endpoint_index(&ep->desc);