FAILED: patch "[PATCH] USB: gadget: f_midi: fixing a possible double-free in f_midi" failed to apply to 4.4-stable tree

The patch below does not apply to the 4.4-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From: "Yavuz, Tuba" <[email protected]>
Date: Fri, 23 Mar 2018 17:00:38 +0000
Subject: [PATCH] USB: gadget: f_midi: fixing a possible double-free in f_midi

It looks like there is a possibility of a double-free vulnerability on an error path of the f_midi_set_alt function in the f_midi driver. If the path is feasible then free_ep_req gets called twice:

         req->complete = f_midi_complete;
         err = usb_ep_queue(midi->out_ep, req, GFP_ATOMIC);
            => ...
             usb_gadget_giveback_request
               =>
                 f_midi_complete (CALLBACK)
                   (inside f_midi_complete, for various cases of status)
                   free_ep_req(ep, req); // first kfree
         if (err) {
                 ERROR(midi, "%s: couldn't enqueue request: %d\n",
                             midi->out_ep->name, err);
                 free_ep_req(midi->out_ep, req); // second kfree
                 return err;
         }

The double-free possibility was introduced with commit ad0d1a058eac ("usb: gadget: f_midi: fix leak on failed to enqueue out requests").

Found by MOXCAFE tool.

index 4eb96b91cc40..e8f35db42394 100644
@@ -404,7 +404,8 @@ static int f_midi_set_alt(struct usb_function *f, unsigned intf, unsigned alt)
 		if (err) {
 			ERROR(midi, "%s: couldn't enqueue request: %d\n",
 				    midi->out_ep->name, err);
-			free_ep_req(midi->out_ep, req);
+			if (req->buf != NULL)
+				free_ep_req(midi->out_ep, req);
 			return err;
 		}
 	}

index c3fbef2bb5db..09f90447fed5 100644
@@ -61,7 +61,9 @@ struct usb_request *alloc_ep_req(struct usb_ep *ep, size_t len);
 /* Frees a usb_request previously allocated by alloc_ep_req() */
 static inline void free_ep_req(struct usb_ep *ep, struct usb_request *req)
 {
+	WARN_ON(req->buf == NULL);
 	kfree(req->buf);
+	req->buf = NULL;
 	usb_ep_free_request(ep, req);
 }