CVE-2025-38108 in Linux
Summary
by MITRE • 07/03/2025
In the Linux kernel, the following vulnerability has been resolved:
net_sched: red: fix a race in __red_change()
Gerrard Tai reported a race condition in RED, whenever SFQ perturb timer fires at the wrong time.
The race is as follows:
CPU 0 CPU 1 [1]: lock root
[2]: qdisc_tree_flush_backlog()
[3]: unlock root
| | [5]: lock root
| [6]: rehash
| [7]: qdisc_tree_reduce_backlog()
| [4]: qdisc_put()
This can be abused to underflow a parent's qlen.
Calling qdisc_purge_queue() instead of qdisc_tree_flush_backlog() should fix the race, because all packets will be purged from the qdisc before releasing the lock.
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Analysis
by VulDB Data Team • 04/17/2026
The vulnerability identified as CVE-2025-38108 represents a critical race condition within the Linux kernel's networking subsystem, specifically affecting the RED (Random Early Detection) queuing discipline implementation. This flaw exists in the net_sched component of the kernel, which manages network traffic scheduling and queuing mechanisms. The issue manifests when the SFQ (Stochastic Fairness Queuing) perturb timer fires at an inopportune moment, creating a timing window where concurrent operations can lead to inconsistent state management within the queuing system.
The race condition occurs during the execution of the __red_change() function when multiple CPU cores attempt to manipulate the same queuing structures simultaneously. The scenario unfolds with CPU 0 acquiring a lock on the root qdisc structure, performing qdisc_tree_flush_backlog() operations, and then releasing the lock, while CPU 1 concurrently attempts to acquire the same lock, perform rehash operations, and modify the backlog. This interleaving creates a situation where the qdisc_put() operation on CPU 0 can underflow the parent's qlen counter, leading to potential memory corruption or system instability. The vulnerability is classified under CWE-362, which specifically addresses race conditions in software systems.
The operational impact of this vulnerability extends beyond simple memory corruption, as it can lead to denial of service conditions where network traffic becomes severely disrupted or completely blocked. Network administrators and system operators may experience unexpected network failures, packet loss, or complete network stack crashes when this race condition is triggered. The vulnerability is particularly concerning in high-throughput environments where SFQ perturb timer activity is frequent, as the timing window for exploitation becomes more probable. According to ATT&CK framework domain T1499, this vulnerability could be leveraged to perform resource exhaustion attacks or system disruption, while T1595.001 indicates potential for network infrastructure compromise through manipulation of queuing mechanisms.
The proposed fix involves replacing qdisc_tree_flush_backlog() with qdisc_purge_queue() in the affected code path, which ensures that all packets are purged from the qdisc before releasing the lock mechanism. This approach eliminates the race condition by preventing concurrent access to the queuing structures during cleanup operations. The solution addresses the fundamental issue of incomplete synchronization between different CPU cores when manipulating shared network queuing resources. System administrators should prioritize applying kernel updates that include this fix, particularly in production environments where network reliability is critical. The mitigation strategy aligns with security best practices for preventing race conditions in kernel-level networking code, ensuring proper synchronization primitives are maintained throughout the queuing discipline lifecycle management.