Efficient and secure in-process isolation is in great demand, as evidenced in the shift towards JavaScript and the recent revival of memory protection keys. Yet, state-of-the-art systems do not offer strong security or struggle with frequent domain crossings and oftentimes intrusive kernel modifications. We propose Donky, an efficient hardware-software codesign for strong in-process isolation based on dynamic memory protection domains. The two components of our design are a secure software

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... work and a non-intrusive hardware extension. We facilitate domain switches entirely in userspace, thus minimizing switching overhead as well as kernel complexity. We show the versatility of Donky in three realistic use cases, secure V8 sandboxing, software vaults, and untrusted third-party libraries. We provide an open-source implementation on a RISC-V Ariane CPU and an Intel-MPK-based emulation mode for x86. We evaluate the security and performance of our implementation for RISC-V synthesized on an FPGA. We also evaluate the performance on x86 and show why our new design is more secure than Intel MPK. Donky does not impede the runtime of in-domain computation. Cross-domain switches are 16-116x faster than regular process context switches. Fully protecting the mbedTLS cryptographic operations has a 4 % overhead.