APRON: Authenticated and Progressive System Image Renovation.

The integrity and availability of an operating system are important to securely use a computing device. Conventional schemes focus on how to prevent adversaries from corrupting the operating system or how to detect such corruption. However, how to recover the device from such corruption securely and efficiently is overlooked, resulting in lengthy system downtime with integrity violation and unavailability. In this paper, we propose APRON, a novel scheme to renovate a corrupt or outdated operating system image securely and progressively. APRON concurrently and selectively repairs any invalid blocks on demand during and after the system boot, effectively minimizing the system downtime needed for a recovery. APRON verifies whether requested blocks are valid in the kernel using a signed Merkle hash tree computed over the valid, up-to-date system image. If they are invalid, it fetches corresponding blocks from a reliable source, verifies them, and replaces the requested blocks with the fetched ones. Once the system boots up, APRON runs a background thread to eventually renovate any other non-requested invalid blocks. Our evaluation shows that APRON has short downtime: it outperforms conventional recovery mechanisms by up to 28x. It runs real-world applications with an average runtime overhead of 9% during the renovation and with negligible overhead (0.01%) once the renovation is completed.