[PATCH] usb: gadget: dummyhcd: Fix use-after-free in dummy_free_request

[Xin Zhao <xnzhao@google.com>]

DummyHCD assume when dummy_free_request is called, the request
is already detached from request queues. It is correct in most
cases.
But when DummyHCD is detached from gadget configfs with pending
requests and some requests are still in pending queue,
dummy_free_request would free them directly.
Later on, dummy_udc_stop would iterate pending queue to release
the requests again.

Stacktrace for dummy_free_reqeust
```
kfree(const void * x) (slub.c:4200)
dummy_free_request(struct usb_ep * _ep, struct usb_request * _req) (dummy_hcd.c:691)
usb_ep_free_request(struct usb_ep * ep, struct usb_request * req) (core.c:201)
functionfs_unbind(struct ffs_data * ffs) (f_fs.c:1894)
ffs_func_unbind(struct usb_function * f) (f_fs.c:3614)
purge_configs_funcs(struct gadget_info * gi) (configfs.c:1303)
configfs_composite_unbind(struct usb_gadget * gadget) (configfs.c:1528)
usb_gadget_remove_driver(struct usb_udc * udc) (core.c:1436)
usb_gadget_unregister_driver(struct usb_gadget_driver * driver) (core.c:1585)
unregister_gadget(struct gadget_info * gi) (configfs.c:281)
gadget_dev_desc_UDC_store(struct config_item * item) (configfs.c:308)
flush_write_buffer(struct file * file, struct configfs_buffer * buffer, size_t count) (file.c:251)
```

Stacktrace of use-after-free
```
list_del_init(struct list_head * entry) (list.h:204)
nuke(struct dummy * dum, struct dummy_ep * ep) (dummy_hcd.c:344)
stop_activity(struct dummy * dum) (dummy_hcd.c:366)
dummy_udc_stop(struct usb_gadget * g) (dummy_hcd.c:1032)
usb_gadget_udc_stop(struct usb_udc * udc) (core.c:1141)
usb_gadget_remove_driver(struct usb_udc * udc) (core.c:1437)
usb_gadget_unregister_driver(struct usb_gadget_driver * driver) (core.c:1585)
unregister_gadget(struct gadget_info * gi) (configfs.c:281)
gadget_dev_desc_UDC_store(struct config_item * item) (configfs.c:308)
flush_write_buffer(struct file * file, struct configfs_buffer * buffer, size_t count) (file.c:251)
configfs_write_file(struct file * file)
```

Signed-off-by: Xin Zhao <xnzhao@google.com>
---
 drivers/usb/gadget/udc/dummy_hcd.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/usb/gadget/udc/dummy_hcd.c b/drivers/usb/gadget/udc/dummy_hcd.c
index 899ac9f9c279..afead69d7487 100644
--- a/drivers/usb/gadget/udc/dummy_hcd.c
+++ b/drivers/usb/gadget/udc/dummy_hcd.c
@@ -679,7 +679,11 @@ static void dummy_free_request(struct usb_ep *_ep, struct usb_request *_req)
 	}
 
 	req = usb_request_to_dummy_request(_req);
-	WARN_ON(!list_empty(&req->queue));
+	if (!list_empty(&req->queue)) {
+		WARN_ON(1);
+		return;
+	}
+
 	kfree(req);
 }
 
-- 
2.39.1.519.gcb327c4b5f-goog