CVE-2021-46986

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().