CVE-2026-31647: Linux Kernel Race Condition Vulnerability

CVE-2026-31647 Overview

CVE-2026-31647 is a race condition vulnerability in the Linux kernel's idpf (Intel Infrastructure Data Path Function) network driver. The vulnerability involves improper spinlock nesting when handling asynchronous virtual channel (VC) operations, specifically affecting systems running with PREEMPT_RT (Real-Time preemption) configurations. The flaw occurs because the async handler incorrectly takes a BH (bottom half) spinlock while already holding a raw spinlock from the completion structure, creating an invalid wait context.

Critical Impact
Local attackers with low privileges can trigger kernel deadlocks or system instability, leading to denial of service on systems using Intel idpf network drivers with real-time kernel configurations.

Affected Products

Linux Kernel versions 6.8.x through 6.8.13
Linux Kernel version 6.9
Linux Kernel versions 7.0-rc1 through 7.0-rc7

Discovery Timeline

April 24, 2026 - CVE-2026-31647 published to NVD
April 27, 2026 - Last updated in NVD database

Technical Details for CVE-2026-31647

Vulnerability Analysis

The vulnerability resides in the idpf driver's mailbox task handler (idpf_mbx_task) which processes asynchronous virtual channel messages. When the idpf_recv_mb_msg function processes incoming messages, it holds a raw spinlock from the completion structure (&x->wait#34). Subsequently, when idpf_mac_filter_async_handler is invoked, it attempts to acquire the mac_filter_list_lock using spin_lock_bh(), which expects a different locking context.

This creates a lock ordering violation where a level-2 lock (raw spinlock) is held while attempting to acquire a level-3 lock (BH spinlock). On PREEMPT_RT kernels, this is particularly problematic because raw spinlocks are actual spinning locks while regular spinlocks become sleeping locks, making the nesting invalid.

Root Cause

The root cause is the improper use of the completion structure's internal raw spinlock for synchronization across both the completion mechanism and the async handler callback path. The completion API's internal lock (&x->wait) operates at a different preemption level than the BH spinlock (mac_filter_list_lock) used in the MAC filter async handler.

The fix involves switching from using the completion's raw spinlock to a dedicated local lock within the idpf_vc_xn struct. This separation ensures that complete() and complete_all() calls occur outside the critical section, eliminating the lock hierarchy violation.

Attack Vector

The vulnerability requires local access to the system with the ability to trigger network operations that exercise the idpf driver's mailbox processing path.

The attack involves triggering concurrent mailbox operations on systems with Intel idpf network interfaces, particularly when PREEMPT_RT configurations are enabled. The kernel workqueue idpf-0000:83:00.0-mbx processes these operations, and when the idpf_mac_filter_async_handler function at offset +0xe9 attempts to acquire the vport_config->mac_filter_list_lock, the invalid wait context triggers a kernel warning and potential deadlock condition.

Detection Methods for CVE-2026-31647

Indicators of Compromise

Kernel log messages containing "BUG: Invalid wait context" with references to the idpf module
Stack traces showing idpf_mac_filter_async_handler and idpf_recv_mb_msg in the call chain
System hangs or unresponsive network interfaces on systems with Intel idpf drivers

Detection Strategies

Monitor kernel logs (dmesg) for lock ordering warnings involving the idpf driver
Implement automated alerting for kernel lockdep warnings on production systems
Deploy kernel tracing to detect abnormal spinlock acquisition patterns in network driver paths

Monitoring Recommendations

Enable kernel lockdep debugging on test systems to proactively identify lock ordering issues
Monitor system stability metrics on hosts with Intel idpf network interfaces
Track kernel worker thread states for idpf-*-mbx workqueues for stuck or hung conditions

How to Mitigate CVE-2026-31647

Immediate Actions Required

Update affected Linux kernel versions to patched releases as soon as available
Consider temporarily disabling PREEMPT_RT configurations on systems with idpf network interfaces if real-time guarantees are not critical
Monitor affected systems closely for kernel warnings or instability until patches are applied

Patch Information

The Linux kernel development team has released patches addressing this vulnerability across multiple kernel branches. The fix modifies the idpf_vc_xn struct to use a local lock instead of relying on the completion's internal spinlock. Patches are available through the following kernel commits:

Workarounds

Avoid using PREEMPT_RT kernel configurations on systems with Intel idpf network interfaces until patched
If possible, use alternative network drivers or interface configurations that do not rely on the idpf driver
Implement watchdog mechanisms to detect and recover from potential deadlock conditions

bash

# Check current kernel version for vulnerability
uname -r

# Verify idpf driver is in use
lsmod | grep idpf

# Check kernel preemption model
cat /boot/config-$(uname -r) | grep PREEMPT

# Monitor for lock warnings in kernel logs
dmesg | grep -i "invalid wait context"