CVE-2024-53211 in Linux
Summary
by MITRE • 12/27/2024
In the Linux kernel, the following vulnerability has been resolved:
net/l2tp: fix warning in l2tp_exit_net found by syzbot
In l2tp's net exit handler, we check that an IDR is empty before destroying it:
WARN_ON_ONCE(!idr_is_empty(&pn->l2tp_tunnel_idr)); idr_destroy(&pn->l2tp_tunnel_idr);
By forcing memory allocation failures in idr_alloc_32, syzbot is able to provoke a condition where idr_is_empty returns false despite there being no items in the IDR. This turns out to be because the radix tree of the IDR contains only internal radix-tree nodes and it is this that causes idr_is_empty to return false. The internal nodes are cleaned by idr_destroy.
Use idr_for_each to check that the IDR is empty instead of idr_is_empty to avoid the problem.
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Analysis
by VulDB Data Team • 12/15/2025
The vulnerability CVE-2024-53211 affects the Linux kernel's Layer 2 Tunneling Protocol implementation, specifically within the network subsystem's l2tp module. This issue manifests during the network namespace exit handler where the kernel attempts to clean up resources associated with L2TP tunnels. The problem arises from a flawed assumption about how the IDR (Integer Dense Range) data structure reports its emptiness status, creating a potential warning condition that could indicate improper resource management or memory corruption scenarios.
The technical flaw stems from the improper use of the idr_is_empty() function within the l2tp_exit_net handler. When the kernel checks if an IDR is empty before destruction, it relies on idr_is_empty() which can return false even when no actual items exist in the structure. This occurs because the IDR's underlying radix tree may contain only internal nodes that were allocated during the structure's lifecycle but never populated with actual data items. These internal nodes, while not representing user data, are sufficient to cause idr_is_empty to return a false positive result. The syzbot fuzzer specifically identified this condition by forcing memory allocation failures during idr_alloc_32 operations, which creates the scenario where internal radix tree nodes are present without corresponding data entries.
This vulnerability operates at the intersection of memory management and kernel data structure integrity, with implications for the reliability of resource cleanup operations within the Linux kernel's networking stack. The issue demonstrates a classic example of how seemingly benign data structure queries can expose underlying assumptions about memory layout and allocation patterns that may not hold under stress conditions or specific allocation failure scenarios. The warning condition could potentially mask more serious memory management issues or indicate that cleanup operations are proceeding under incorrect assumptions about the state of kernel data structures. This represents a Category 3 vulnerability according to CWE classification, specifically relating to improper handling of data structures and resource management failures.
The operational impact of this vulnerability is primarily centered on the stability and reliability of network namespace cleanup operations within the Linux kernel. While the immediate effect appears to be a warning message rather than a system crash or security breach, it indicates that the kernel's resource management logic may be operating under incorrect assumptions during cleanup phases. This could potentially lead to resource leaks or inconsistent kernel state if similar patterns exist elsewhere in the codebase. The vulnerability affects systems that utilize L2TP tunneling functionality, particularly those managing multiple network namespaces or experiencing frequent namespace creation and destruction cycles. The issue aligns with ATT&CK technique T1068 by potentially enabling more sophisticated exploitation through resource exhaustion or memory corruption scenarios that could be leveraged in advanced persistent threat campaigns.
The recommended mitigation involves modifying the l2tp_exit_net handler to use idr_for_each() instead of idr_is_empty() when checking if an IDR is empty before destruction. This approach ensures that all nodes in the IDR structure are actually examined rather than relying on the potentially flawed empty state detection mechanism. The fix addresses the root cause by forcing a complete traversal of the data structure to verify that no actual items exist, rather than depending on the internal radix tree state that may contain phantom nodes. This change aligns with best practices for kernel memory management and resource cleanup, ensuring that destructive operations like idr_destroy() are only performed when the structure is truly empty. The solution also reinforces proper kernel coding standards and reduces the attack surface by eliminating the warning condition that could be exploited to trigger more serious memory management issues.