CVE-2026-31680: Linux Kernel Race Condition Vulnerability

CVE-2026-31680 Overview

A use-after-free vulnerability has been discovered in the Linux kernel's IPv6 flowlabel implementation. The vulnerability exists in the ip6fl_seq_show() function, which walks the global flowlabel hash under the seq-file RCU read-side lock and prints fl->opt->opt_nflen when an option block is present. Exclusive flowlabels currently free fl->opt as soon as fl->users drops to zero in fl_release(). However, the surrounding struct ip6_flowlabel remains visible in the global hash table until later garbage collection removes it and fl_free_rcu() finally tears it down.

A concurrent /proc/net/ip6_flowlabel reader can therefore race that early kfree() and dereference freed option state, triggering a crash in ip6fl_seq_show(). This race condition allows a local attacker with low privileges to potentially achieve code execution or cause system instability.

Critical Impact
Local attackers can exploit this race condition to cause kernel crashes or potentially achieve privilege escalation by triggering a use-after-free condition in the IPv6 flowlabel subsystem.

Affected Products

Linux Kernel (multiple stable branches affected)

Discovery Timeline

2026-04-25 - CVE-2026-31680 published to NVD
2026-04-27 - Last updated in NVD database

Technical Details for CVE-2026-31680

Vulnerability Analysis

This vulnerability is a classic use-after-free race condition occurring in the Linux kernel's IPv6 flowlabel management code. The core issue stems from a lifetime mismatch between the fl->opt option block and its enclosing struct ip6_flowlabel structure.

When a flowlabel's reference count (fl->users) reaches zero, the fl_release() function immediately frees the associated option block via kfree(). However, the parent ip6_flowlabel structure continues to exist in the global hash table until RCU garbage collection eventually removes it and fl_free_rcu() completes the teardown process.

During this window between the early kfree() of fl->opt and the eventual removal of the flowlabel from the hash table, a concurrent reader accessing /proc/net/ip6_flowlabel can traverse the hash table under RCU protection and attempt to dereference the already-freed option pointer. This results in accessing freed memory, which can cause immediate kernel crashes or potentially be exploited for code execution.

Root Cause

The root cause is improper synchronization between memory deallocation and RCU-protected data structure visibility. The option block fl->opt is freed prematurely in fl_release() while the containing ip6_flowlabel structure remains accessible to RCU readers. The fix defers the fl->opt deallocation to fl_free_rcu(), ensuring the option block lifetime matches that of the enclosing flowlabel structure.

Attack Vector

The attack requires local access to the system with low privileges. An attacker can exploit this vulnerability by:

Creating IPv6 flowlabels with exclusive options attached
Triggering the release of the flowlabel (causing fl->opt to be freed)
Simultaneously reading /proc/net/ip6_flowlabel to race against the kfree() operation
Winning the race to cause ip6fl_seq_show() to dereference the freed fl->opt pointer

The vulnerability is exploitable through timing manipulation, where repeated attempts to read the proc interface while triggering flowlabel releases can reliably trigger the use-after-free condition. Due to the nature of the vulnerability, no external network access is required.

Detection Methods for CVE-2026-31680

Indicators of Compromise

Kernel crash logs showing null pointer dereference or invalid memory access in ip6fl_seq_show()
Kernel oops or panic messages referencing the IPv6 flowlabel subsystem (net/ipv6/ip6_flowlabel.c)
Unexpected system reboots or kernel crashes when accessing /proc/net/ip6_flowlabel
KASAN (Kernel Address Sanitizer) reports indicating use-after-free in flowlabel code paths

Detection Strategies

Monitor kernel logs for crash reports originating from ip6fl_seq_show() or related flowlabel functions
Deploy KASAN-enabled debug kernels in test environments to detect memory safety violations
Implement system call monitoring for suspicious patterns of flowlabel creation and proc filesystem access
Use eBPF-based monitoring to detect anomalous access patterns to /proc/net/ip6_flowlabel

Monitoring Recommendations

Enable kernel crash dump collection (kdump) to capture and analyze kernel crashes for forensic investigation
Monitor process activity for repeated rapid access to /proc/net/ip6_flowlabel combined with socket operations
Configure alerting on kernel oops or panic events in system monitoring infrastructure
Review audit logs for privilege escalation attempts following kernel instability

How to Mitigate CVE-2026-31680

Immediate Actions Required

Apply the official kernel patches from the Linux kernel stable branches immediately
Prioritize patching systems where untrusted local users have access
Consider restricting access to /proc/net/ip6_flowlabel via filesystem permissions as a temporary measure
Monitor affected systems for signs of exploitation attempts

Patch Information

The Linux kernel developers have released patches across multiple stable branches to address this vulnerability. The fix defers the fl->opt option block deallocation to fl_free_rcu(), ensuring the option lifetime matches the enclosing ip6_flowlabel structure lifetime while RCU readers can still reach it.

Official patch commits are available from the following kernel git repositories:

Workarounds

Restrict local user access to systems where patching cannot be immediately applied
Limit access to /proc/net/ip6_flowlabel using appropriate file permissions for non-root users
Consider disabling IPv6 flowlabel functionality if not required for operations (may impact IPv6 QoS features)
Implement mandatory access control (SELinux/AppArmor) policies to restrict proc filesystem access

bash

# Restrict access to ip6_flowlabel proc entry as a temporary workaround
chmod 600 /proc/net/ip6_flowlabel

# Verify current kernel version and check if patch is needed
uname -r

# Update kernel to patched version (example for Debian/Ubuntu)
apt update && apt upgrade linux-image-$(uname -r)

# Reboot to apply new kernel
systemctl reboot