Andrea Di Dio (Vrije Universiteit Amsterdam), Koen Koning (Intel), Herbert Bos (Vrije Universiteit Amsterdam), Cristiano Giuffrida (Vrije Universiteit Amsterdam)

Despite nearly decade-long mitigation efforts in academia and industry, the
community is yet to find a practical solution to the Rowhammer vulnerability.
Comprehensive software mitigations require complex changes to commodity systems, yielding significant run-time overhead and deterring practical
adoption. Hardware mitigations, on the other hand, have generally grown more robust and efficient, but are difficult to deploy on commodity systems. Until recently, ECC memory implemented by the memory controller on server platforms seemed to provide the best of both worlds: use hardware features already on commodity systems to efficiently turn Rowhammer into a denial-of-service attack vector. Unfortunately, researchers have recently shown that attackers can perform one-bit-at-a-time memory templating and mount ECC-aware Rowhammer attacks.

In this paper, we reconsider ECC memory as an avenue for Rowhammer mitigations
and show that not all hope is lost. In particular, we show that it is feasible to devise a software-based design to both efficiently and effectively harden commodity ECC memory against ECC-aware Rowhammer attacks. To support this claim, we present Copy-on-Flip (CoF), an ECC-based software mitigation which uses a combination of memory _migration_ and _offlining_ to stop Rowhammer attacks on commodity server systems in a practical way. The key idea is to let the operating system interpose on all the error correction events and offline the vulnerable victim page as soon as the attacker has successfully templated a sufficient number of bit flips---while transparently migrating the victim data to a new page. We present a CoF prototype on Linux, where we also show it is feasible to operate simple memory management changes to support migration for the relevant user and kernel memory pages. Our evaluation shows CoF incurs low performance and memory overhead, while significantly reducing the Rowhammer attack surface. On typical benchmarks such as SPEC CPU2017 and Google Chrome, CoF reports a $<1.5%$ overhead, and, on extreme I/O-intensive scenarios (saturated nginx), up to $sim11%$.

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