Youssef Tobah, University of Michigan; Andrew Kwong, UNC Chapel Hill; Ingab Kang, University of Michigan; Daniel Genkin, Georgia Tech; Kang G. Shin, University of Michigan
Rowhammer is an increasingly threatening vulnerability that grants an attacker the ability to flip bits in memory without directly accessing them. Despite efforts to mitigate Rowhammer via software and defenses built directly into DRAM modules, more recent generations of DRAM are actually more susceptible to malicious bit-flips than their predecessors. This phenomenon has spawned numerous exploits, showing how Rowhammer acts as the basis for various vulnerabilities that target sensitive structures, such as Page Table Entries (PTEs) or opcodes, to grant control over a victim machine.
However, in this paper, we consider Rowhammer as a more general vulnerability, presenting a novel exploit vector for Rowhammer that targets particular code patterns. We show that if victim code is designed to return benign data to an unprivileged user, and uses nested pointer dereferences, Rowhammer can flip these pointers to gain arbitrary read access in the victim's address space. Furthermore, we identify gadgets present in the Linux kernel, and demonstrate an end-to-end attack that precisely flips a targeted pointer. To do so we developed a number of improved Rowhammer primitives, including kernel memory massaging, Rowhammer synchronization, and testing for kernel flips, which may be of broader interest to the Rowhammer community. Compared to prior works' leakage rate of .3 bits/s, we show that such gadgets can be used to read out kernel data at a rate of 82.6 bits/s.
By targeting code gadgets, this work expands the scope and attack surface exposed by Rowhammer. It is no longer sufficient for software defenses to selectively pad previously exploited memory structures in flip-safe memory, as any victim code that follows the pattern in question must be protected.
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