Identifying Linux Kernel Instability Due to Poor RCU Synchronization

TL;DR

This paper investigates how improper RCU synchronization in the Linux kernel, especially in ICE network drivers, causes instability, memory fragmentation, and potential security vulnerabilities, emphasizing the need for explicit synchronize_rcu() calls.

Contribution

It identifies a specific driver-level synchronization issue in Linux kernel RCU hash tables and proposes best practices to prevent kernel instability and memory leaks.

Findings

Improper RCU synchronization leads to stale entries and memory fragmentation.

Removing VF entries without proper sync causes delayed memory reclamation.

Explicit synchronize_rcu() calls improve kernel stability and safety.

Abstract

Read-Copy-Update (RCU) is widely used in the Linux kernel to manage concurrent access to shared data structures.However, improper synchronization when removing RCU protected hash table entries can lead to stale pointers, inconsistent lookups, and critical use after free (UAF) vulnerabilities. This paper investigates a driver-level synchronization issue arising from the omission of explicit synchronize_rcu() calls during hash table updates, using a discovered weakness in the Intel ICE network drivers Virtual Function (VF) management. Previous kernel vulnerabilities, such as a bug in the Reliable Datagram Sockets (RDS) subsystem, show how improper RCU synchronization can directly cause kernel crashes. Experimental results demonstrate that removing VF entries without proper synchronization leaves transient stale entries, delays memory reclamation, and results in significant memory fragmentation under rapid insert/delete workloads. RCU hash tables are widely deployed in Linux kernel subsystems such as networking, virtualization, and file systems; improper synchronization can cause memory fragmentation, kernel instability, and out-of-memory (OOM) conditions. Mitigations are proposed, recommending explicit insertion of synchronize_rcu() calls to ensure timely and safe memory reclamation. These findings reinforce established best practices for RCU synchronization, highlighting their importance for maintaining kernel stability and memory safety. Keywords: RCU, kernel synchronization, hash tables, ICE driver, memory fragmentation, use-after-free