CVE-2026-31419: Linux Kernel Use-After-Free Vulnerability

CVE-2026-31419 Overview

CVE-2026-31419 is a use-after-free vulnerability in the Linux kernel's network bonding driver, specifically in the bond_xmit_broadcast() function. The vulnerability occurs due to a race condition in how the driver handles socket buffer (skb) reuse during broadcast transmissions when concurrent slave enslave/release operations mutate the slave list.

The bond_xmit_broadcast() function reuses the original skb for the last slave (determined by bond_is_last_slave()) and clones it for others. However, concurrent slave enslave/release operations can mutate the slave list during RCU-protected iteration, changing which slave is "last" mid-loop. This causes the original skb to be double-consumed, resulting in a double-free condition that can lead to kernel crashes and potential security exploitation.

Critical Impact

This use-after-free vulnerability in the Linux kernel bonding driver can lead to kernel crashes, denial of service, and potentially arbitrary code execution in kernel context through memory corruption exploitation.

Affected Products

• Linux kernel versions with vulnerable bonding driver implementation
• Systems utilizing network bonding/teaming in broadcast mode
• Network infrastructure devices running affected Linux kernel versions

Discovery Timeline

• April 13, 2026 - CVE-2026-31419 published to NVD
• April 13, 2026 - Last updated in NVD database

Technical Details for CVE-2026-31419

Vulnerability Analysis

The vulnerability exists in the bond_xmit_broadcast() function within drivers/net/bonding/bond_main.c. The bonding driver implements a zero-copy optimization where the original socket buffer is reused for the last slave in the list rather than being cloned. This optimization relies on the bond_is_last_slave() function to determine which slave is the final one in the iteration.

The fundamental flaw lies in the race condition between the RCU-protected iteration over the slave list and concurrent slave enslave/release operations. While RCU (Read-Copy-Update) protects against use-after-free of the slave structures themselves, it does not prevent the list from being mutated during iteration. When the slave list changes mid-loop, the determination of which slave is "last" becomes unstable, potentially causing the original skb to be consumed multiple times.

Root Cause

The root cause is a Time-of-Check Time-of-Use (TOCTOU) race condition in the bond_is_last_slave() check. The function dynamically evaluates whether the current slave is the last one in the list, but this evaluation can return different results between iterations if the slave list is concurrently modified.

When a slave is added or removed during the broadcast loop:

1. An iteration may correctly identify slave A as non-last and clone the skb
2. A concurrent slave release removes slave B (which was the actual last slave)
3. The next iteration now sees slave A as the last slave
4. The original skb, already consumed in a previous iteration, is used again
5. This triggers a double-free when the skb is eventually freed

Attack Vector

The vulnerability can be triggered through the network stack when transmitting packets over a bonding interface configured in broadcast mode. An attacker with the ability to:

1. Send network traffic through a bond interface in broadcast mode
2. Simultaneously trigger slave enslave/release operations (requiring appropriate privileges)

Can cause the use-after-free condition. The KASAN crash dump from the CVE description demonstrates the exploitation path:

The crash occurs in skb_clone() when attempting to access memory at address ffff888100ef8d40, which belongs to a freed 224-byte region in the skbuff_head_cache slab. The memory state shows the region has been marked as freed (fa poison pattern), but the code attempts to read from it, triggering the KASAN slab-use-after-free detection.

The call trace shows the path from user-space (__sys_sendto) through the IPv6 UDP stack, ultimately reaching bond_xmit_broadcast() where the vulnerability manifests.

Detection Methods for CVE-2026-31419

Indicators of Compromise

• KASAN slab-use-after-free warnings in kernel logs referencing skb_clone or bond_xmit_broadcast
• Kernel panics or crashes with call traces involving bond_main.c and skbuff.c
• Memory corruption patterns in skbuff_head_cache slab allocations
• Unexpected network bonding interface failures or instability

Detection Strategies

• Enable KASAN (Kernel Address Sanitizer) on systems where feasible to detect memory access violations
• Monitor kernel logs for bonding driver error messages or warnings during high network activity
• Implement network monitoring for unusual packet loss patterns on bonded interfaces
• Deploy kernel debugging tools to track slab allocator behavior on critical systems

Monitoring Recommendations

• Configure syslog alerting for kernel memory corruption warnings
• Monitor system stability metrics for unexpected crashes or reboots
• Track bonding interface state changes that could indicate exploitation attempts
• Implement network baseline monitoring to detect anomalous traffic patterns through bonded interfaces

How to Mitigate CVE-2026-31419

Immediate Actions Required

• Apply the official kernel patches from the Linux kernel stable branches immediately
• If patching is not immediately possible, consider temporarily disabling broadcast mode on bonding interfaces
• Restrict access to bonding interface configuration to reduce attack surface
• Monitor affected systems for signs of exploitation until patches are applied

Patch Information

The Linux kernel maintainers have released patches to address this vulnerability. The fix replaces the racy bond_is_last_slave() check with a simple index comparison (i + 1 == slaves_count) against a pre-snapshot slave count taken via READ_ONCE() before the loop. This preserves the zero-copy optimization for the last slave while making the "last" determination stable against concurrent list mutations.

Official patches are available from the following kernel git commits:

• Kernel Git Commit #2884bf7
• Kernel Git Commit #d4cc7e4
• Kernel Git Commit #f5b9465

Workarounds

• Switch bonding interfaces from broadcast mode to alternative modes (active-backup, 802.3ad, balance-xor) if operationally feasible
• Implement network segmentation to limit exposure of systems with vulnerable bonding configurations
• Restrict bonding configuration privileges to prevent unauthorized slave modifications during operation
• Enable KASAN in development/testing environments to catch exploitation attempts early

# Temporary workaround: Change bonding mode from broadcast to active-backup
# WARNING: This will cause brief network interruption
echo "active-backup" > /sys/class/net/bond0/bonding/mode

# Alternatively, restrict bonding configuration access
chmod 600 /sys/class/net/bond0/bonding/*

# Monitor for exploitation attempts in kernel logs
dmesg -w | grep -E "(KASAN|slab-use-after-free|bond_xmit)"