Leaky MDU: ARM Memory Disambiguation Unit Uncovered and Vulnerabilities Exposed

Memory Disambiguation Unit (MDU) is widely used on modern processors to speculatively execute load instructions and improve pipeline performance. Given that the MDU design details on ARM processors are not available to the public, it is unclear whether there are any security vulnerabilities associated with its MDU. In this paper, we first reverse engineer the undocumented features of ARM MDU, then we discover three potential user-privilege attacks to leak secret data via MDU: cross-process attack that allows users to communicate through a convert channel, cross-domain attack that leaks kernel information and a new variant of inner-process and inter-processes Spectre attacks. These attacks pose serious security challenges as they can bypass both all the known countermeasures against cache side-channel attacks and those against transient execution attacks. Potential mitigation against the proposed MDU-based attacks are also discussed.