Linux Kernel FUSE Block Filesystem Livelock Vulnerability in File Release Handling

A livelock vulnerability has been identified in the Linux kernel's handling of file releases in the FUSE block filesystem (fuseblk). This issue arises when a file is closed before all asynchronous I/O (AIO) write operations are completed. The FUSE server threads then end up waiting for responses from themselves, creating a deadlock situation. This vulnerability affects several versions of the Linux kernel.

Impact

This vulnerability can lead to a livelock situation, where the system becomes unresponsive due to FUSE server threads being stuck in a synchronous file release process, unable to handle queued FUSE commands.

Reproduction

The vulnerability can be reproduced by running the generic/323 test case against a FUSE block filesystem server. This test opens a file, starts multiple AIO writes to it, and then closes the file before the writes are finished. As a result, the AIO threads wait for responses from the FUSE server, which is also tied up handling its own synchronous file release, creating a livelock.

Remediation

The vulnerability has been addressed by modifying the FUSE file release process to always use asynchronous file puts when closing files, preventing the FUSE server from getting stuck in a synchronous release operation.