FSCQ is the first file system with a machine-checkable proof (using the Coq proof assistant) that its implementation meets its specification and whose specification includes crashes. FSCQ provably avoids bugs that have plagued previous file systems, such as performing disk writes without sufficient barriers or forgetting to zero out directory blocks. If a crash happens at an inopportune time, these bugs can lead to data loss. FSCQ's theorems prove that, under any sequence of crashes followed by reboots, FSCQ will recover the file system correctly without losing data. To state FSCQ's theorems, this paper introduces the Crash Hoare logic (CHL), which extends traditional Hoare logic with a crash condition, a recovery procedure, and logical address spaces for specifying disk states at different abstraction levels. CHL also reduces the proof effort for developers through proof automation. Using CHL, we developed, specified, and proved the correctness of the FSCQ file system. Although FSCQ's design is relatively simple, experiments with FSCQ running as a user-level file system show that it is sufficient to run Unix applications with usable performance. FSCQ's specifications and proofs required significantly more work than the implementation, but the work was manageable even for a small team of a few researchers.
The security of many applications relies on the kernel being secure, but history suggests that kernel vulnerabilities are routinely discovered and exploited. In particular, exploitable vulnerabilities in kernel modules are common. This paper proposes LXFI, a system which isolates kernel modules from the core kernel so that vulnerabilities in kernel modules cannot lead to a privilege escalation attack. To safely give kernel modules access to complex kernel APIs, LXFI introduces the notion of API integrity, which captures the set of contracts assumed by an interface. To partition the privileges within a shared module, LXFI introduces module principals. Programmers specify principals and API integrity rules through capabilities and annotations. Using a compiler plugin, LXFI instruments the generated code to grant, check, and transfer capabilities between modules, according to the programmer's annotations. An evaluation with Linux shows that the annotations required on kernel functions to support a new module are moderate, and that LXFI is able to prevent three known privilege-escalation vulnerabilities. Stress tests of a network driver module also show that isolating this module using LXFI does not hurt TCP throughput but reduces UDP throughput by 35%, and increases CPU utilization by 2.2-3.7×.
DFSCQ is the first file system that (1) provides a precise specification for fsync and fdatasync, which allow applications to achieve high performance and crash safety, and (2) provides a machine-checked proof that its implementation meets this specification. DFSCQ's specification captures the behavior of sophisticated optimizations, including logbypass writes, and DFSCQ's proof rules out some of the common bugs in file-system implementations despite the complex optimizations. The key challenge in building DFSCQ is to write a specification for the file system and its internal implementation without exposing internal file-system details. DFSCQ introduces a metadata-prefix specification that captures the properties of fsync and fdatasync, which roughly follows the behavior of Linux ext4. This specification uses a notion of tree sequences-logical sequences of file-system tree states-for succinct description of the possible states after a crash and to describe how data writes can be reordered with respect to metadata updates. This helps application developers prove the crash safety of their own applications, avoiding application-level bugs such as forgetting to invoke fsync on both the file and the containing directory. An evaluation shows that DFSCQ achieves 103 MB/s on large file writes to an SSD and durably creates small files at a rate of 1,618 files per second. This is slower than Linux ext4 (which achieves 295 MB/s for large file writes and 4,977 files/s for small file creation) but much faster than two recent verified file systems, Yggdrasil and FSCQ. Evaluation results from application-level benchmarks, including TPC-C on SQLite, mirror these microbenchmarks.
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