xen-gnt: prevent adding duplicate gnt callbacks

With the current implementation, the callback in the tail of the list
can be added twice, because the check done in
gnttab_request_free_callback is bogus, callback->next can be NULL if
it is the last callback in the list. If we add the same callback twice
we end up with an infinite loop, were callback == callback->next.

Replace this check with a proper one that iterates over the list to
see if the callback has already been added.
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Matt Wilson <msw@amazon.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
CC: stable@vger.kernel.org

drivers/xen/grant-table.c

@@ -730,9 +730,18 @@ void gnttab_request_free_callback(struct gnttab_free_callback *callback,
 				  void (*fn)(void *), void *arg, u16 count)
 {
 	unsigned long flags;
+	struct gnttab_free_callback *cb;
+
 	spin_lock_irqsave(&gnttab_list_lock, flags);
-	if (callback->next)
-		goto out;
+
+	/* Check if the callback is already on the list */
+	cb = gnttab_free_callback_list;
+	while (cb) {
+		if (cb == callback)
+			goto out;
+		cb = cb->next;
+	}
+
 	callback->fn = fn;
 	callback->arg = arg;
 	callback->count = count;