IB/uverbs: Fix checking of userspace object ownership

Commit 9ead190b ("IB/uverbs: Don't serialize with ib_uverbs_idr_mutex")
rewrote how userspace objects are looked up in the uverbs module's
idrs, and introduced a severe bug in the process: there is no checking
that an operation is being performed by the right process any more.
Fix this by adding the missing check of uobj->context in __idr_get_uobj().

Apparently everyone is being very careful to only touch their own
objects, because this bug was introduced in June 2006 in 2.6.18, and
has gone undetected until now.

Cc: stable <stable@kernel.org>
Signed-off-by: Roland Dreier <rolandd@cisco.com>

drivers/infiniband/core/uverbs_cmd.c

@@ -147,8 +147,12 @@ static struct ib_uobject *__idr_get_uobj(struct idr *idr, int id,
 
 	spin_lock(&ib_uverbs_idr_lock);
 	uobj = idr_find(idr, id);
-	if (uobj)
-		kref_get(&uobj->ref);
+	if (uobj) {
+		if (uobj->context == context)
+			kref_get(&uobj->ref);
+		else
+			uobj = NULL;
+	}
 	spin_unlock(&ib_uverbs_idr_lock);
 
 	return uobj;