CVE-2026-23126: Linux Kernel Race Condition Vulnerability

CVE-2026-23126 Overview

CVE-2026-23126 is a race condition vulnerability in the Linux kernel's netdevsim driver affecting operations on the bpf_bound_progs list. The netdevsim driver lacks proper protection mechanisms for concurrent list operations, allowing simultaneous list_add_tail and list_del operations to occur. When nsim_bpf_create_prog() adds entries while nsim_bpf_destroy_prog() simultaneously removes them, list corruption can occur, triggering a kernel crash and system instability.

Critical Impact

This race condition can cause kernel crashes and system denial of service through list corruption in the BPF program management subsystem of the netdevsim driver.

Affected Products

• Linux kernel with netdevsim driver enabled
• Linux kernel version 6.19.0-rc5 (confirmed affected)
• Systems using BPF program offloading with netdevsim

Discovery Timeline

• 2026-02-14 - CVE CVE-2026-23126 published to NVD
• 2026-02-18 - Last updated in NVD database

Technical Details for CVE-2026-23126

Vulnerability Analysis

This vulnerability represents a classic race condition in kernel-space list management. The netdevsim driver, which provides a simulated network device primarily used for testing BPF programs and network configurations, maintains a linked list called bpf_bound_progs to track bound BPF programs. The fundamental issue is the absence of synchronization primitives protecting concurrent access to this shared data structure.

When the kernel's bpf_prog_free_deferred workqueue handler invokes nsim_bpf_destroy_prog() to remove a program from the list, there is no mechanism preventing nsim_bpf_create_prog() from simultaneously adding a new entry. This concurrent modification violates the integrity constraints of the doubly-linked list implementation, causing the kernel's list debugging code at lib/list_debug.c:62 to detect corruption and trigger a BUG assertion.

The crash manifests through the __list_del_entry_valid_or_report function, which validates list pointer consistency before deletion. When corruption is detected, it results in an invalid opcode exception, leading to a kernel panic with the PREEMPT SMP NOPTI configuration.

Root Cause

The root cause is the absence of mutex or spinlock protection around list operations in the netdevsim BPF subsystem. Both nsim_bpf_create_prog() and nsim_bpf_destroy_prog() functions access the bpf_bound_progs list without holding any synchronization primitive, creating a Time-of-Check Time-of-Use (TOCTOU) vulnerability where list pointers can become invalid between the time they are read and when they are used in list manipulation operations.

Attack Vector

The vulnerability is triggered through concurrent BPF program operations on the netdevsim device. An attacker or legitimate user activity that involves rapid creation and destruction of BPF programs bound to a netdevsim interface can race these operations, leading to kernel panic. The attack vector requires local access and the ability to interact with the netdevsim driver through BPF system calls.

The race condition occurs in the events workqueue during deferred BPF program cleanup. When the kernel schedules bpf_prog_free_deferred work items while new programs are being attached, the concurrent list operations can corrupt memory and crash the system.

Detection Methods for CVE-2026-23126

Indicators of Compromise

• Kernel panic messages referencing __list_del_entry_valid_or_report in lib/list_debug.c
• BUG assertions with call traces involving nsim_bpf_destroy_prog in the netdevsim module
• Workqueue crash events in bpf_prog_free_deferred handling
• System instability during BPF program offloading operations on netdevsim devices

Detection Strategies

• Monitor kernel logs for BUG assertions originating from list debugging code paths
• Enable kernel memory debugging options (CONFIG_DEBUG_LIST) to catch list corruption early
• Implement crash dump analysis to identify race conditions in netdevsim BPF operations
• Deploy auditd rules to track BPF system calls targeting netdevsim interfaces

Monitoring Recommendations

• Configure kdump or similar crash dump mechanisms to capture kernel state during failures
• Enable ftrace or eBPF-based tracing on nsim_bpf_create_prog and nsim_bpf_destroy_prog functions
• Monitor system stability metrics on hosts using netdevsim for BPF testing
• Set up alerts for kernel oops or panic events related to list corruption

How to Mitigate CVE-2026-23126

Immediate Actions Required

• Apply the kernel patches from the official Linux kernel git repository
• If patching is not immediately possible, avoid using the netdevsim driver in production environments
• Restrict access to BPF system calls on systems where netdevsim is loaded
• Unload the netdevsim kernel module if not required for testing purposes

Patch Information

The Linux kernel maintainers have released patches that add mutex lock protection to prevent simultaneous addition and deletion operations on the bpf_bound_progs list. The fix introduces proper synchronization around the list operations in both nsim_bpf_create_prog() and nsim_bpf_destroy_prog() functions.

Multiple patch commits are available for different kernel branches:

• Kernel Git Commit 3f560cf
• Kernel Git Commit 68462ec
• Kernel Git Commit b97d5ee
• Kernel Git Commit d77379c
• Kernel Git Commit f1f9cfd

Workarounds

• Unload the netdevsim module using rmmod netdevsim if not actively needed
• Blacklist the netdevsim module by adding blacklist netdevsim to /etc/modprobe.d/blacklist.conf
• Restrict BPF capabilities using seccomp or AppArmor profiles to limit exposure
• Isolate testing environments using netdevsim from production workloads

# Unload netdevsim module and prevent automatic loading
sudo rmmod netdevsim
echo "blacklist netdevsim" | sudo tee /etc/modprobe.d/netdevsim-blacklist.conf
sudo update-initramfs -u