CVE-2024-40920 in Linux
Summary
by MITRE • 07/12/2024
In the Linux kernel, the following vulnerability has been resolved:
net: bridge: mst: fix suspicious rcu usage in br_mst_set_state
I converted br_mst_set_state to RCU to avoid a vlan use-after-free but forgot to change the vlan group dereference helper. Switch to vlan group RCU deref helper to fix the suspicious rcu usage warning.
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Analysis
by VulDB Data Team • 09/17/2025
The vulnerability CVE-2024-40920 addresses a critical issue within the Linux kernel's networking subsystem, specifically within the bridge module's Multiple Spanning Tree Protocol implementation. This flaw resides in the br_mst_set_state function which handles state management for multiple spanning tree instances within bridge networks. The vulnerability represents a subtle but dangerous race condition that could potentially lead to memory corruption and system instability when the kernel processes spanning tree protocol updates in bridge configurations.
The technical root cause stems from an incomplete refactoring of the br_mst_set_state function to utilize RCU (Read-Copy-Update) mechanisms for improved concurrency handling. While the primary function was correctly converted to use RCU to prevent a use-after-free condition involving vlan structures, the developers inadvertently overlooked updating a critical helper function that dereferences vlan group structures. This oversight created a mismatch between the RCU-protected data access patterns and the actual dereference operations, generating suspicious RCU usage warnings during kernel compilation and runtime analysis.
The operational impact of this vulnerability extends beyond simple warning messages as it represents a potential pathway for memory corruption within the kernel's networking stack. When bridge devices process spanning tree updates, particularly during topology changes or configuration modifications, the improper RCU handling could lead to accessing freed memory locations or inconsistent data states. This scenario could manifest as system crashes, data corruption, or in extreme cases, privilege escalation opportunities that attackers might exploit to gain unauthorized access to the system. The vulnerability particularly affects systems running Linux kernels with bridge networking capabilities and multiple spanning tree protocol implementations.
Mitigation strategies for CVE-2024-40920 require immediate kernel updates from vendors that include the fix for the RCU usage pattern in br_mst_set_state. System administrators should prioritize patching affected systems, particularly those running bridge configurations with multiple spanning tree protocol enabled. The fix involves switching the vlan group dereference helper to use the proper RCU dereference mechanism, ensuring consistency between the RCU-protected function and its supporting helper operations. Organizations should also implement monitoring for kernel warnings related to RCU usage patterns and consider implementing network segmentation strategies to limit the attack surface where bridge networking is employed. This vulnerability aligns with CWE-367, indicating a Time-of-Check to Time-of-Use error, and could potentially map to ATT&CK techniques related to privilege escalation through kernel exploitation.