CVE-2025-37798 Overview
CVE-2025-37798 is a vulnerability in the Linux kernel affecting the CoDel (Controlled Delay) and fq_codel (Fair Queuing with Controlled Delay) qdisc (queuing discipline) subsystem. The vulnerability involves the removal of a queue length check (sch->q.qlen) before calling qdisc_tree_reduce_backlog() in both fq_codel_dequeue() and codel_qdisc_dequeue() functions. This change was made after the ->qlen_notify() callbacks were made idempotent, addressing a potential race condition or state inconsistency in the network packet scheduling subsystem.
Critical Impact
Local attackers with low privileges could potentially exploit this vulnerability to achieve high impact on confidentiality, integrity, and availability of affected Linux systems.
Affected Products
- Linux Kernel (multiple versions)
- Linux Kernel 6.15-rc1
- Debian Linux 11.0
Discovery Timeline
- May 2, 2025 - CVE-2025-37798 published to NVD
- November 6, 2025 - Last updated in NVD database
Technical Details for CVE-2025-37798
Vulnerability Analysis
The vulnerability exists in the CoDel (Controlled Delay) Active Queue Management algorithm implementation within the Linux kernel's traffic control subsystem. CoDel is designed to control bufferbloat by managing packet queuing delays in network buffers. The fq_codel variant combines CoDel with fair queuing to provide both delay control and flow isolation.
The core issue relates to the queue length check (sch->q.qlen != 0) that was previously performed before calling qdisc_tree_reduce_backlog() in the dequeue functions. This check was originally implemented to prevent callback invocations when the queue was empty. However, after architectural changes that made all ->qlen_notify() callbacks idempotent (safe to call multiple times with the same result), this check became unnecessary and its removal was required for proper operation.
The vulnerability could potentially lead to inconsistent queue state management, which a local attacker with low privileges could exploit to impact system stability, cause memory corruption, or escalate privileges.
Root Cause
The root cause stems from a design inconsistency in the qdisc callback mechanism. The qdisc_tree_reduce_backlog() function is responsible for notifying parent qdiscs about changes in queue length. Prior to this fix, the conditional check on qlen could lead to situations where the backlog reduction was skipped when it shouldn't have been, or conversely, could be called in unexpected states.
The fix removes the redundant check from both fq_codel_dequeue() and codel_qdisc_dequeue(), relying instead on the idempotent nature of the notification callbacks to handle edge cases safely.
Attack Vector
This is a local attack vector vulnerability requiring low privileges. An attacker with local access to an affected system could potentially:
- Manipulate network traffic or queue operations to trigger the vulnerable code path
- Exploit race conditions in the qdisc dequeue operations
- Cause kernel memory corruption or denial of service conditions
- Potentially escalate privileges by exploiting the inconsistent queue state
The vulnerability requires local access to the system, making it unsuitable for remote exploitation without chaining with another vulnerability that provides initial access.
Detection Methods for CVE-2025-37798
Indicators of Compromise
- Unexpected kernel crashes or panics related to network subsystem operations
- Anomalous behavior in the qdisc or traffic control subsystem
- Memory corruption indicators in kernel logs referencing fq_codel or codel modules
- System instability during high network traffic periods
Detection Strategies
- Monitor kernel logs for errors or warnings related to fq_codel, codel, or qdisc subsystems
- Implement kernel integrity monitoring to detect unexpected modifications to qdisc-related data structures
- Deploy endpoint detection solutions capable of monitoring kernel-level operations
- Review system call patterns for anomalous traffic control (tc) operations
Monitoring Recommendations
- Enable verbose logging for the network traffic control subsystem
- Monitor for processes attempting to manipulate qdisc configurations with elevated privileges
- Implement system call auditing for setsockopt, sendmsg, and related network system calls
- Deploy kernel live patching solutions to ensure rapid remediation
How to Mitigate CVE-2025-37798
Immediate Actions Required
- Update the Linux kernel to a patched version as soon as possible
- Review and restrict local user access to systems running vulnerable kernel versions
- Monitor for any suspicious activity related to network traffic control operations
- Consider disabling fq_codel and codel qdiscs if not required and patching is delayed
Patch Information
Multiple patches have been released by the Linux kernel maintainers to address this vulnerability. The following kernel commits contain the fix:
- Linux Kernel Commit 2f9761a94b
- Linux Kernel Commit 342debc121
- Linux Kernel Commit 4d55144b12
- Linux Kernel Commit 7a742a9506
- Linux Kernel Commit 829c49b6b2
- Linux Kernel Commit a57fe60ef4
- Linux Kernel Commit cc71a757da
- Linux Kernel Commit e73c838c80
- Linux Kernel Commit eda741fe15
Debian users should refer to the Debian LTS Announcement May 2025 and Debian LTS Announcement October 2025 for distribution-specific updates.
Workarounds
- If immediate patching is not possible, consider switching to alternative qdisc algorithms (e.g., pfifo_fast, htb)
- Restrict local user access to minimize the attack surface
- Implement network namespace isolation to limit qdisc manipulation capabilities
- Monitor and audit traffic control configuration changes
# Check current qdisc configuration
tc qdisc show
# Temporarily replace fq_codel with pfifo_fast as workaround
tc qdisc replace dev eth0 root pfifo_fast
# Verify the change
tc qdisc show dev eth0
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
