CVE-2026-31546: Linux Kernel Use-After-Free Vulnerability

CVE-2026-31546 Overview

CVE-2026-31546 is a NULL pointer dereference vulnerability in the Linux kernel's bonding driver, specifically within the bond_debug_rlb_hash_show function. The vulnerability occurs when the Receive Load Balancing (RLB) hash-table display function attempts to access slave information without first checking if the slave pointer is NULL. This condition can arise when rlb_clear_slave intentionally keeps hash-table entries on the rx_hashtbl_used_head list with a NULL slave pointer when no replacement slave is available.

Critical Impact
Local attackers with low privileges can trigger a kernel panic and system crash by accessing the debugfs interface for bonding RLB hash information when slave entries exist without assigned slaves.

Affected Products

Linux Kernel versions from 2.6.38
Linux Kernel 7.0 release candidates (rc1 through rc7)
Systems using network bonding with RLB mode and debugfs enabled

Discovery Timeline

April 24, 2026 - CVE-2026-31546 published to NVD
April 28, 2026 - Last updated in NVD database

Technical Details for CVE-2026-31546

Vulnerability Analysis

This vulnerability is classified as CWE-476 (NULL Pointer Dereference). The flaw exists in the bond_debug_rlb_hash_show function located in drivers/net/bonding/bond_debugfs.c. When iterating through the RLB hash-table entries via the used-list, the function directly accesses client_info->slave without validating that the pointer is non-NULL.

The inconsistency is notable because other iterators in bond_alb.c already handle this NULL-slave state safely. For example, rlb_update_client returns early when !client_info->slave, while functions like rlb_req_update_slave_clients, rlb_clear_slave, and rlb_rebalance compare slave values before accessing them. The lb_req_update_subnet_clients function continues if the slave is NULL.

Root Cause

The root cause is a missing NULL check in the bond_debug_rlb_hash_show function. When rlb_clear_slave removes a slave from the bonding configuration but no replacement slave is available, it intentionally preserves hash-table entries on the rx_hashtbl_used_head list with their slave pointer set to NULL. The debugfs display function was not designed to handle this legitimate NULL state, leading to a kernel NULL pointer dereference when reading the debugfs file.

Attack Vector

The attack requires local access to the system with low privileges. An attacker can trigger the vulnerability by reading the bonding RLB hash debugfs file when the bonding driver has entries with NULL slave pointers. The crash occurs through the following call chain:

The kernel crash trace shows the dereference occurring at RIP bond_debug_rlb_hash_show (line 41 in bond_debugfs.c), triggered through the debugfs read path: entry_SYSCALL_64_after_hwframe → do_syscall_64 → ksys_read → vfs_read → full_proxy_read → seq_read → seq_read_iter → bond_debug_rlb_hash_show.

The fix adds a NULL check before accessing the slave pointer and prints "(none)" for entries without an assigned slave, maintaining debugfs functionality while preventing the crash.

Detection Methods for CVE-2026-31546

Indicators of Compromise

Kernel panic messages containing "BUG: kernel NULL pointer dereference" with address 0000000000000000
Stack traces showing bond_debug_rlb_hash_show in the RIP register
System crashes when accessing /sys/kernel/debug/bonding/*/rlb_hash_table files
Kernel log entries referencing drivers/net/bonding/bond_debugfs.c:41

Detection Strategies

Monitor kernel logs for NULL pointer dereference crashes originating from bonding driver components
Implement auditd rules to track access to bonding debugfs files under /sys/kernel/debug/bonding/
Use kernel live patching detection tools to verify patch status on production systems
Deploy endpoint detection rules for abnormal debugfs access patterns

Monitoring Recommendations

Enable kernel crash dump collection (kdump) to capture detailed information if exploitation occurs
Configure syslog forwarding for real-time analysis of kernel error messages
Monitor for unusual user access to debugfs bonding interfaces
Implement file integrity monitoring on kernel modules related to network bonding

How to Mitigate CVE-2026-31546

Immediate Actions Required

Update to patched kernel versions that include the NULL check fix
Restrict access to debugfs by unmounting /sys/kernel/debug if not required for operations
Limit local user access to systems running vulnerable kernel versions
Consider disabling network bonding RLB mode if not actively required

Patch Information

The Linux kernel maintainers have released patches across multiple stable branches. The fix adds a NULL check before accessing client_info->slave and prints "(none)" for entries without an assigned slave.

Available patches:

Workarounds

Unmount debugfs to prevent access to the vulnerable interface: umount /sys/kernel/debug
Restrict debugfs mount permissions using filesystem ACLs or SELinux policies
Disable bonding RLB mode by switching to alternative bonding modes (e.g., balance-rr, active-backup)
Implement kernel module parameter restrictions to limit bonding driver functionality

bash

# Temporary workaround: Unmount debugfs to prevent exploitation
umount /sys/kernel/debug

# Alternative: Restrict debugfs mount with limited permissions
mount -o remount,mode=0700 /sys/kernel/debug

# Verify bonding mode (if RLB is not required, consider switching modes)
cat /sys/class/net/bond0/bonding/mode