Swivel: Hardening WebAssembly against Spectre


Shravan Narayan and Craig Disselkoen, UC San Diego; Daniel Moghimi, Worcester Polytechnic Institute and UC San Diego; Sunjay Cauligi, Evan Johnson, and Zhao Gang, UC San Diego; Anjo Vahldiek-Oberwagner, Intel Labs; Ravi Sahita, Intel; Hovav Shacham, UT Austin; Dean Tullsen and Deian Stefan, UC San Diego


We describe Swivel, a new compiler framework for hardening WebAssembly (Wasm) against Spectre attacks. Outside the browser, Wasm has become a popular lightweight, in-process sandbox and is, for example, used in production to isolate different clients on edge clouds and function-as-a-service platforms. Unfortunately, Spectre attacks can bypass Wasm's isolation guarantees. Swivel hardens Wasm against this class of attacks by ensuring that potentially malicious code can neither use Spectre attacks to break out of the Wasm sandbox nor coerce victim code—another Wasm client or the embedding process—to leak secret data.

We describe two Swivel designs, a software-only approach that can be used on existing CPUs, and a hardware-assisted approach that uses extension available in Intel® 11th generation CPUs. For both, we evaluate a randomized approach that mitigates Spectre and a deterministic approach that eliminates Spectre altogether. Our randomized implementations impose under 10.3% overhead on the Wasm-compatible subset of SPEC 2006, while our deterministic implementations impose overheads between 3.3% and 240.2%. Though high on some benchmarks, Swivel's overhead is still between 9× and 36.3× smaller than existing defenses that rely on pipeline fences.

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@inproceedings {272260,
author = {Shravan Narayan and Craig Disselkoen and Daniel Moghimi and Sunjay Cauligi and Evan Johnson and Zhao Gang and Anjo Vahldiek-Oberwagner and Ravi Sahita and Hovav Shacham and Dean Tullsen and Deian Stefan},
title = {Swivel: Hardening {WebAssembly} against Spectre},
booktitle = {30th USENIX Security Symposium (USENIX Security 21)},
year = {2021},
isbn = {978-1-939133-24-3},
pages = {1433--1450},
url = {https://www.usenix.org/conference/usenixsecurity21/presentation/narayan},
publisher = {USENIX Association},
month = aug

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