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UBC Theses and Dissertations

Secure file system versioning at the block level Wires, Jacob Taylor


Information is capital; disk space is a mere commodity. Versioning file systems offer an appealing storage model that prevents users from unintentionally deleting or overwriting important data by transparently retaining old versions. However, improving storage reliability by adding versioning to a file system is problematic in two important ways. First, the complexity of file systems and the operating systems in which they reside leaves data vulnerable to bugs and viruses, even when versioning is added. Second, the mission-critical nature of file systems makes users and OS vendors justifiably hesitant to adopt new file system features like versioning, regardless of the potential benefits they might provide. This thesis presents VDisk, a block layer system capable of providing file-grain versioning to existing, unmodified file systems. VDisk features a novel division of labor to enhance security and reliability. Write-access to versioned data is restricted to two very simple, reliable, file system agnostic components: a block logger and a log cleaner. These crucial components are isolated in a virtual machine, where they are protected from the errors and attacks that plague operating systems. More complicated, untrusted, read-only utilities operate in user space. These utilities, which are free to use sophisticated, off-the-shelf tools not appropriate for trusted kernels, support version browsing and reconstruction without degrading system reliability. VDisk employs a policy-driven approach to block reclamation. A retention policy specifies a set of constraints that describe which file versions must be retained and which need not be. A user-space tool periodically invokes the secure cleaner by submitting a set of delete requests along with a proof that these requests satisfy the retention policy. The secure cleaner verifies the proof and reclaims the specified blocks if applicable. Experimental results show that the cleaner is capable of reclaiming more than 80% of logged data.

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