Microarchitectural Security of AWS Firecracker VMM: Conclusion, Acknowledgments, and References

13 Jun 2024


(1) Zane Weissman, Worcester Polytechnic Institute Worcester, MA, USA {zweissman@wpi.edu};

(2) Thomas Eisenbarth, University of Lübeck Lübeck, S-H, Germany {thomas.eisenbarth@uni-luebeck.de};

(3) Thore Tiemann, University of Lübeck Lübeck, S-H, Germany {t.tiemann@uni-luebeck.de};

(4) Berk Sunar, Worcester Polytechnic Institute Worcester, MA, USA {sunar@wpi.edu}.


Cloud technologies constantly shift to meet the needs of their customers. At the same time, CSPs aim for maximizing efficiency and profit, which incentivizes serverless CSPs to over-commit available compute resources. While this is reasonable from an economic perspective, the resulting system behavior can be disastrous in the context of microarchitectural attacks that exploit shared hardware resources. In the past few years, the microarchitectural threat landscape changed frequently and rapidly. There mitigations that work reasonably well to prevent many attacks, but they often lead to significant performance costs, which forces CSPs to find a tradeoff between economic value and security. Furthermore, some microarchitectural attacks simply are not hindered by existing mitigations. The CSP customers have little control over the microarchitectural defenses deployed and must trust their providers to keep up with the pace of microarchitectural attack and mitigation development. Defense-in-depth requires security at every level, from the microcode to VMM to container. Each system must be considered as a whole, as some protections at one system level may open a vulnerabilities at another.

We showed that default countermeasures as they are recommended for the Firecracker VMM are insufficient to meet its isolation goals. In fact, many of the tested attack vectors showed leakage while countermeasures where in place. We identified the Medusa cache indexing/block write variant as an attack vector that only works across VMs, i. e. with additional isolation mechanisms in place. Additionally, we showed that disabling SMT–an expensive mitigation technique recommended and performed by AWS–does not provide full protection against Medusa variants. The aforementioned Medusa variant, and Spectre-PHT are still capable of leaking information between cloud tenants even if SMT is disabled, as long as the attacker and target threads keep competing for hardware resources of the same physical CPU core. Unfortunately this is inevitably the case in high-density serverless environments. In the present, serverless CSPs must remain vigilant in keeping firmware up-to-date and employing all possible defenses against microarchitectural attacks. Users must not only trust their CSPs of choice to keep their systems up-to-date and properly configured, but also be aware that some microarchitectural vulnerabilities, particularly certain Spectre variants, are still able to cross containment boundaries. Furthermore, processor designs continue to evolve and speculative and out-of-order execution remain important factors in improving performance from generation to generation. So, it is unlikely that we have seen the last of new microarchitectural vulnerabilities, as the recent wave of newly discovered attacks [36, 47, 53] shows.


This work was supported by the German Research Foundation (DFG) under Grants No. 439797619 and 456967092, by the German Federal Ministry for Education and Research (BMBF) under Grants SASVI and SILGENTAS, by the National Science Foundation (NSF) under Grant CNS-2026913, and in part by a grant from the Qatar National Research Fund.


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