usb: dwc3: gadget: Free gadget structure only after freeing endpoints

In the Linux kernel, the following vulnerability has been resolved:

usb: dwc3: gadget: Free gadget structure only after freeing endpoints

As part of commit e81a7018d93a ("usb: dwc3: allocate gadget structure

dynamically") the dwc3_gadget_release() was added which will free

the dwc->gadget structure upon the device's removal when

usb_del_gadget_udc() is called in dwc3_gadget_exit().

However, simply freeing the gadget results a dangling pointer

situation: the endpoints created in dwc3_gadget_init_endpoints()

have their dep->endpoint.ep_list members chained off the list_head

anchored at dwc->gadget->ep_list. Thus when dwc->gadget is freed,

the first dwc3_ep in the list now has a dangling prev pointer and

likewise for the next pointer of the dwc3_ep at the tail of the list.

The dwc3_gadget_free_endpoints() that follows will result in a

use-after-free when it calls list_del().

This was caught by enabling KASAN and performing a driver unbind.

The recent commit 568262bf5492 ("usb: dwc3: core: Add shutdown

callback for dwc3") also exposes this as a panic during shutdown.

There are a few possibilities to fix this. One could be to perform

a list_del() of the gadget->ep_list itself which removes it from

the rest of the dwc3_ep chain.

Another approach is what this patch does, by splitting up the

usb_del_gadget_udc() call into its separate "del" and "put"

components. This allows dwc3_gadget_free_endpoints() to be

called before the gadget is finally freed with usb_put_gadget().