It is not legal. Making backup copies for yourself works as a fair use defense of copyright infringement but DRM breaking is a separate matter; that's why that part of the DMCA is reviled, it prevents people from doing what copyright otherwise permits them to do.
There is an exemption for breaking DRM on ebooks to enable their use with assistive technologies used by people with disabilities. There are some audiovisual exemptions for educational use that encompass "multimedia ebooks" but only for specific purposes.
