usb: gadget: f_hid: fix: Prevent accessing released memory

commit aa65d11a upstream.

When we unlock our spinlock to copy data to user we may get
disabled by USB host and free the whole list of completed out
requests including the one from which we are copying the data
to user memory.

To prevent from this let's remove our working element from
the list and place it back only if there is sth left when we
finish with it.

Fixes: 99c51500 ("usb: gadget: hidg: register OUT INT endpoint for SET_REPORT")
Tested-by: David Lechner <david@lechnology.com>
Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
[bwh: Backported to 3.16: adjust filename, context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/usb/gadget/f_hid.c

@@ -197,6 +197,13 @@ static ssize_t f_hidg_read(struct file *file, char __user *buffer,
 	/* pick the first one */
 	list = list_first_entry(&hidg->completed_out_req,
 				struct f_hidg_req_list, list);
+
+	/*
+	 * Remove this from list to protect it from beign free()
+	 * while host disables our function
+	 */
+	list_del(&list->list);
+
 	req = list->req;
 	count = min_t(unsigned int, count, req->actual - list->pos);
 	spin_unlock_irqrestore(&hidg->spinlock, flags);
@@ -212,16 +219,21 @@ static ssize_t f_hidg_read(struct file *file, char __user *buffer,
 	 * call, taking into account its current read position.
 	 */
 	if (list->pos == req->actual) {
-		spin_lock_irqsave(&hidg->spinlock, flags);
-		list_del(&list->list);
 		kfree(list);
-		spin_unlock_irqrestore(&hidg->spinlock, flags);
 
 		req->length = hidg->report_length;
 		ret = usb_ep_queue(hidg->out_ep, req, GFP_KERNEL);
-		if (ret < 0)
+		if (ret < 0) {
+			free_ep_req(hidg->out_ep, req);
 			return ret;
 		}
+	} else {
+		spin_lock_irqsave(&hidg->spinlock, flags);
+		list_add(&list->list, &hidg->completed_out_req);
+		spin_unlock_irqrestore(&hidg->spinlock, flags);
+
+		wake_up(&hidg->read_queue);
+	}
 
 	return count;
 }
@@ -471,6 +483,7 @@ static void hidg_disable(struct usb_function *f)
 {
 	struct f_hidg *hidg = func_to_hidg(f);
 	struct f_hidg_req_list *list, *next;
+	unsigned long flags;
 
 	usb_ep_disable(hidg->in_ep);
 	hidg->in_ep->driver_data = NULL;
@@ -478,10 +491,13 @@ static void hidg_disable(struct usb_function *f)
 	usb_ep_disable(hidg->out_ep);
 	hidg->out_ep->driver_data = NULL;
 
+	spin_lock_irqsave(&hidg->spinlock, flags);
 	list_for_each_entry_safe(list, next, &hidg->completed_out_req, list) {
+		free_ep_req(hidg->out_ep, list->req);
 		list_del(&list->list);
 		kfree(list);
 	}
+	spin_unlock_irqrestore(&hidg->spinlock, flags);
 }
 
 static int hidg_set_alt(struct usb_function *f, unsigned intf, unsigned alt)