Although many defense mechanisms against buffer overflow attacks have been proposed, buffer overflow vulnerability in software is still one of the most prevalent vulnerabilities exploited. This paper proposes a micro-architecture based defense mechanism against buffer overflow attacks. As buffer overflow attack leads to a compromised return address, our approach is to provide a software transparent micro-architectural support for return address integrity checking. By keeping an uncompromised copy of the return address separate from the activation record in run-time stack, the return address compromised by a buffer overflow attack can be detected at run time. Since extra copies of return addresses are already found in the return address stack (RAS) for return address prediction in most high-performance microprocessors, this paper considers augmenting the RAS in speculative superscalar processors for return address integrity checking. The new mechanism provides 100% accurate return address prediction as well as integrity checking for return addresses. Hence, it enhances system performance in addition to preventing a buffer overflow attack.