How to fix a leaking bath tub overflow drain?
If the leak is occuring where the overflow attaches to the tub you may be able to seal it. Remove the screws that hold the overflow cover in place. Depending on the style it may also contain the tub drain lever. With the cover removed you may be able to see the contact point between the tub and the seal. Try to fill the entire sealing surface with "Plumbers Putty". Replace the cover as it will pull the overflow pipe and the tub together. If that doesn't work try this product http://www.theplumbinginfo.com/remove-replace-repair-bath-tub-overflow/. I haven't tried it but it looks like it beats tearing walls apart. As an alternative open up the wall behind the tub and after the repair leave an access panel so ot will be easier next time.