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We have implemented a secure network file system called SUNDR that guarantees the integrity of data even when malicious parties control the server. SUNDR splits storage functionality between two untrusted components, a block store and a consistency server. The block store holds all file data and most metadata. Without interpreting metadata, it presents a simple interface for clients to store variable-sized data blocks and later retrieve them by cryptographic hash.The consistency server implements a novel protocol that guarantees close-to-open consistency whenever users see each other's updates. The protocol roughly consists of users exchanging version-stamped digital signatures of block server metadata, though a number of subtleties arise in efficiently supporting concurrent clients and groupwritable files. We have proven the protocol's security under basic cryptographic assumptions. Without somehow producing signed messages valid under a user's (or the superuser's) public key, an attacker cannot tamper with a user's files-even given control of the servers and network. Despite this guarantee, SUNDR performs within a reasonable factor of existing insecure network file systems.

Asbestos, a new prototype operating system, provides novel labeling and isolation mechanisms that help contain the effects of exploitable software flaws. Applications can express a wide range of policies with Asbestos's kernel-enforced label mechanism, including controls on inter-process communication and systemwide information flow. A new event process abstraction provides lightweight, isolated contexts within a single process, allowing the same process to act on behalf of multiple users while preventing it from leaking any single user's data to any other user. A Web server that uses Asbestos labels to isolate user data requires about 1.5 memory pages per user, demonstrating that additional security can come at an acceptable cost.

Decentralized Information Flow Control (DIFC) [24] is an approach to security that allows application writers to control how data flows between the pieces of an application and the outside world. As applied to privacy, DIFC allows untrusted software to compute with private data while trusted security code controls the release of that data. As applied to integrity, DIFC allows trusted code to protect untrusted software from unexpected malicious inputs. In either case, only bugs in the trusted code, which tends to be small and isolated, can lead to security violations.We present Flume, a new DIFC model and system that applies at the granularity of operating system processes and standard OS abstractions (e.g., pipes and file descriptors). Flume eases DIFC's use in existing applications and allows safe interaction between conventional and DIFC-aware processes. Flume runs as a user-level reference monitor on Linux. A process confined by Flume cannot perform most system calls directly; instead, an interposition layer replaces system calls with IPC to the reference monitor, which enforces data flow policies and performs safe operations on the process's behalf. We ported a complex Web application (MoinMoin wiki) to Flume, changing only 2% of the original code. The Flume version is roughly 30-40% slower due to overheads in our current implementation but supports additional security policies impossible without DIFC.

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