Secure file system versioning at the block level

In typical file systems, valuable data is vulnerable to being accidentally or maliciously deleted or overwritten. Versioning file systems protect data from accidents by transparently retaining old versions, but do less well in protecting data from malicious attack. These systems remain vulnerable to attackers who gain unauthorized access to prune old file versions, who bypass the file system to directly manipulate storage, or who exploit bugs in any part of the operating system. This paper presents VDisk, a secure, block-level versioning system that adds file-grain versioning to a standard, unmodified file system. VDisk consists of a set of untrusted user-mode tools and a trusted, secure kernel that is implemented within an isolated Xen virtual machine domain. The secure kernel is designed to be simple and thus trustworthy. This kernel logs file-system updates to a secure log, exports a read-only view of the log to the rest of the system and securely removes unwanted versions from the log. Secure cleaning is implemented in a two-level manner. An untrusted, user-mode cleaner selects log entries for reclamation and submits cleaning requests to the trusted VDisk kernel along with a proof that the request satisifies the device's version-retention policy. The secure kernel verifies the proof and updates the log.