CVE-2025-38083: Linux Kernel Race Condition Vulnerability

CVE-2025-38083 Overview

CVE-2025-38083 is a race condition vulnerability in the Linux kernel's network scheduler (net_sched) subsystem, specifically within the PRIO qdisc (queuing discipline) implementation. The vulnerability exists in the prio_tune() function and can be triggered when the SFQ (Stochastic Fairness Queueing) perturb timer fires at an inopportune moment, creating a time-of-check time-of-use (TOCTOU) condition that can lead to queue length underflow.

Critical Impact

This race condition can be exploited to underflow a parent's queue length (qlen), potentially causing denial of service conditions on affected Linux systems through network scheduler manipulation.

Affected Products

• Linux Kernel (multiple versions)
• Linux Kernel 6.16-rc1
• Debian Linux 11.0

Discovery Timeline

• June 20, 2025 - CVE-2025-38083 published to NVD
• December 17, 2025 - Last updated in NVD database

Technical Details for CVE-2025-38083

Vulnerability Analysis

This vulnerability is classified as CWE-362 (Race Condition). The flaw resides in the prio_tune() function within the Linux kernel's PRIO qdisc implementation. The race condition occurs between two concurrent operations on the network scheduler's queue structures, where improper synchronization allows one CPU to release a lock and proceed with operations while another CPU modifies the same data structures.

The race window exists between the qdisc_tree_flush_backlog() call and the subsequent qdisc_put() operation on one CPU, while another CPU simultaneously performs rehashing and qdisc_tree_reduce_backlog() operations. This timing vulnerability allows an attacker with local access to manipulate the queue length counter, potentially causing an underflow condition in the parent qdisc's qlen value.

Root Cause

The root cause of this vulnerability is insufficient synchronization in the prio_tune() function when handling qdisc backlog operations. The original implementation calls qdisc_tree_flush_backlog() while holding the root lock, then releases the lock before calling qdisc_put(). This creates a window where another CPU can acquire the lock and perform rehash and reduce_backlog operations on the same queue structures.

The fix addresses this by replacing the qdisc_tree_flush_backlog() call with qdisc_purge_queue(), which ensures all packets are purged from the qdisc before releasing the lock, thereby eliminating the race window.

Attack Vector

The attack exploits a race condition in the PRIO qdisc tuning operation:

1. CPU 0 acquires the root lock and begins flushing the backlog
2. CPU 0 releases the root lock after the flush operation
3. CPU 1 acquires the root lock during the window between unlock and qdisc_put()
4. CPU 1 performs rehash and reduce_backlog operations
5. CPU 0 completes qdisc_put(), but the queue state is now inconsistent

This sequence can underflow the parent's qlen counter, as the race allows both CPUs to modify queue accounting independently without proper coordination.

Detection Methods for CVE-2025-38083

Indicators of Compromise

• Unexpected kernel warnings or panics related to qdisc or net_sched subsystems
• Abnormal network scheduler behavior including packet drops or queue stalls
• Kernel log messages indicating qlen underflow or negative queue length values

Detection Strategies

• Monitor kernel logs for warnings related to prio_tune, qdisc_tree_flush_backlog, or qdisc_tree_reduce_backlog
• Implement kernel tracing (ftrace) on net_sched functions to detect anomalous timing patterns
• Deploy SentinelOne Singularity Platform for real-time kernel-level monitoring and behavioral analysis

Monitoring Recommendations

• Enable audit logging for network configuration changes involving qdisc manipulation
• Monitor for unusual patterns in network scheduler configuration via tc command usage
• Implement system monitoring for unexpected CPU spikes in kernel network scheduling code paths

How to Mitigate CVE-2025-38083

Immediate Actions Required

• Update the Linux kernel to a patched version as soon as available from your distribution
• Review and apply kernel patches from the official kernel.org stable branches
• Limit local user access to systems until patches are applied, as this is a local attack vector
• Monitor for Debian LTS security announcements and apply updates promptly

Patch Information

Multiple patches have been released to address this vulnerability across various kernel branches. The fix modifies prio_tune() to use qdisc_purge_queue() instead of qdisc_tree_flush_backlog(), ensuring proper packet purging before lock release. Patches are available from the following kernel commits:

• Kernel Patch Commit 20f68e6a9e41
• Kernel Patch Commit 3aaa7c01cf19
• Kernel Patch Commit 4483d8b91275
• Kernel Patch Commit 46c15c9d0f65

Debian users should reference the Debian LTS Security Announcements for distribution-specific updates.

Workarounds

• Restrict access to network configuration tools (tc, ip) to trusted administrators only
• Limit local user accounts on systems where the PRIO qdisc is actively used
• Consider disabling or avoiding PRIO qdisc configurations until patching is complete
• Implement namespace isolation to limit exposure of network scheduler manipulation capabilities

# Restrict tc command to root only
chmod 750 /sbin/tc

# Verify current qdisc configurations
tc qdisc show

# Monitor for qdisc changes
auditctl -w /sbin/tc -p x -k network_scheduler