CVE-2026-23255: Linux Kernel Race Condition Vulnerability

CVE-2026-23255 Overview

A race condition vulnerability has been identified in the Linux kernel's /proc/net/ptype implementation. The vulnerability stems from improper RCU (Read-Copy-Update) protection in the ptype_seq_show() and ptype_seq_next() functions, which can lead to RCU stalls and potential system instability.

The vulnerability occurs because ptype_seq_show() reads pt->dev to obtain the device name without proper memory barriers while running under rcu_read_lock(). Concurrent writers can remove a packet_type structure and clear pt->dev without waiting for an RCU grace period, creating a race condition that violates RCU safety rules.

Critical Impact

This race condition in the Linux kernel's network subsystem can cause RCU stalls, potentially leading to system hangs or denial of service conditions on affected systems.

Affected Products

• Linux Kernel (versions with vulnerable /proc/net/ptype implementation)

Discovery Timeline

• 2026-03-18 - CVE CVE-2026-23255 published to NVD
• 2026-03-19 - Last updated in NVD database

Technical Details for CVE-2026-23255

Vulnerability Analysis

This vulnerability represents a race condition in the Linux kernel's network protocol type enumeration interface. The core issue lies in the interaction between readers accessing /proc/net/ptype and concurrent writers modifying packet type structures.

When a process reads from /proc/net/ptype, the kernel invokes ptype_seq_show() under RCU read-side protection. However, the implementation fails to properly protect against concurrent modifications to the pt->dev pointer. While the packet_type structure itself is correctly freed after an RCU grace period, the clearing of pt->dev happens without waiting for readers to complete, creating a window where readers may access stale or invalid device pointers.

Additionally, ptype_seq_next() lacks proper READ_ONCE() annotations when reading list.next values, which can lead to undefined behavior under compiler optimizations and concurrent modifications.

Root Cause

The root cause is a violation of RCU synchronization rules in the kernel's /proc/net/ptype implementation. The ptype_seq_show() function accesses pt->dev without proper memory barriers, while concurrent writers can modify this pointer without respecting the RCU grace period. The fix introduces a ptype_iter_state structure that safely records the device pointer during iteration, ensuring that ptype_seq_show() operates on a stable snapshot rather than a potentially changing value.

Attack Vector

The vulnerability can be triggered through concurrent access patterns involving:

1. A process reading from /proc/net/ptype to enumerate registered protocol handlers
2. Concurrent network subsystem operations that modify packet type registrations
3. The race window between RCU read-side critical sections and writer-side modifications

While exploitation requires specific timing conditions, the vulnerability can manifest as RCU stalls leading to system unresponsiveness. Local attackers with read access to /proc/net/ptype could potentially trigger denial of service conditions by repeatedly reading the file while manipulating network protocol registrations.

Detection Methods for CVE-2026-23255

Indicators of Compromise

• RCU stall warnings appearing in kernel logs (dmesg) referencing ptype_seq_show or related functions
• System hangs or unresponsiveness during network configuration changes
• Soft lockup warnings in kernel logs with stack traces involving /proc/net/ptype access
• Unusual latency or timeouts when processes read from /proc/net/ptype

Detection Strategies

• Monitor kernel logs for RCU stall messages using log aggregation tools with pattern matching for ptype_seq functions
• Implement kernel watchdog monitoring to detect soft lockups that may indicate exploitation attempts
• Use kernel tracing tools (ftrace, BPF) to monitor access patterns to /proc/net/ptype
• Deploy intrusion detection rules to flag abnormal access frequency to network proc filesystem entries

Monitoring Recommendations

• Enable kernel RCU debugging options (CONFIG_RCU_STALL_COMMON) in development and testing environments
• Configure syslog alerting for RCU stall warnings with priority escalation
• Implement system health monitoring that tracks kernel message rates for anomaly detection
• Consider deploying eBPF-based monitoring for /proc/net/ptype access patterns in sensitive environments

How to Mitigate CVE-2026-23255

Immediate Actions Required

• Review kernel version and determine if your system is affected by checking for the presence of unpatched /proc/net/ptype handling code
• Apply available kernel patches from the stable kernel tree
• Schedule system maintenance windows for kernel updates and subsequent reboots
• Monitor affected systems for RCU stall warnings until patches can be applied

Patch Information

The vulnerability has been addressed in kernel patches available through the official kernel.org stable tree. The fix introduces a ptype_iter_state structure to safely carry the device pointer alongside the sequence iterator, ensuring proper RCU protection during enumeration.

Relevant kernel commits:

• Kernel Commit 589a530ae44d
• Kernel Commit f613e8b4afea

Workarounds

• Restrict access to /proc/net/ptype using file permissions or security modules (SELinux/AppArmor) to limit potential attack surface
• Reduce frequency of operations that modify packet type registrations during periods when /proc/net/ptype may be accessed
• Consider disabling unnecessary network monitoring tools that regularly poll /proc/net/ptype until patches are applied
• Implement process isolation to limit which processes can access network proc filesystem entries

# Restrict access to /proc/net/ptype using mount namespace isolation
# Note: This is a workaround and not a substitute for proper patching
chmod 400 /proc/net/ptype