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

As part of commit e81a7018 ("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 568262bf ("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().

Fixes: e81a7018 ("usb: dwc3: allocate gadget structure dynamically")
Reviewed-by: NPeter Chen <peter.chen@kernel.org>
Signed-off-by: NJack Pham <jackp@codeaurora.org>
Link: https://lore.kernel.org/r/20210501093558.7375-1-jackp@codeaurora.orgSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/dwc3/gadget.c

@@ -4062,8 +4062,9 @@ int dwc3_gadget_init(struct dwc3 *dwc)
 
 void dwc3_gadget_exit(struct dwc3 *dwc)
 {
-	usb_del_gadget_udc(dwc->gadget);
+	usb_del_gadget(dwc->gadget);
 	dwc3_gadget_free_endpoints(dwc);
+	usb_put_gadget(dwc->gadget);
 	dma_free_coherent(dwc->sysdev, DWC3_BOUNCE_SIZE, dwc->bounce,
 			  dwc->bounce_addr);
 	kfree(dwc->setup_buf);