CVE-2021-47125 in Linux
Summary
by MITRE • 03/15/2024
In the Linux kernel, the following vulnerability has been resolved:
sch_htb: fix refcount leak in htb_parent_to_leaf_offload
The commit ae81feb7338c ("sch_htb: fix null pointer dereference on a null new_q") fixes a NULL pointer dereference bug, but it is not correct.
Because htb_graft_helper properly handles the case when new_q is NULL, and after the previous patch by skipping this call which creates an inconsistency : dev_queue->qdisc will still point to the old qdisc, but cl->parent->leaf.q will point to the new one (which will be noop_qdisc, because new_q was NULL). The code is based on an assumption that these two pointers are the same, so it can lead to refcount leaks.
The correct fix is to add a NULL pointer check to protect qdisc_refcount_inc inside htb_parent_to_leaf_offload.
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Analysis
by VulDB Data Team • 01/07/2025
The vulnerability described in CVE-2021-47125 represents a critical reference count leak within the Linux kernel's traffic control subsystem, specifically affecting the htb (Hierarchical Token Bucket) queuing discipline implementation. This issue manifests in the sch_htb module where improper handling of null pointer conditions during queuing discipline grafting operations creates inconsistencies between internal data structures that manage reference counting for network queuing disciplines. The flaw stems from a previous attempted fix that introduced a new inconsistency rather than resolving the underlying problem correctly.
The technical root cause lies in the htb_parent_to_leaf_offload function where the kernel fails to properly validate null pointer conditions before incrementing reference counts. When new_q becomes NULL during queuing discipline operations, the system maintains inconsistent state between dev_queue->qdisc which still points to the old queuing discipline and cl->parent->leaf.q which points to the new noop_qdisc. This discrepancy creates a scenario where reference counting operations occur without proper validation, leading to memory leaks that persist throughout the system's operation. The vulnerability operates at the kernel level within the networking subsystem, specifically targeting the traffic control framework that manages packet scheduling and queuing behavior.
The operational impact of this vulnerability extends beyond simple memory consumption issues as it can lead to progressive resource exhaustion that degrades system performance and potentially causes system instability. Network operations relying on htb queuing discipline may experience increased latency, packet loss, or complete service disruption as reference count leaks accumulate over time. The vulnerability affects systems utilizing the Linux kernel's traffic control capabilities, particularly those implementing hierarchical queuing structures for network traffic management. Attackers could potentially exploit this weakness to cause denial of service conditions through resource exhaustion, making it particularly concerning in network infrastructure and server environments where traffic control is extensively used.
The fix for CVE-2021-47125 requires implementing a proper NULL pointer check within the htb_parent_to_leaf_offload function before executing qdisc_refcount_inc operations. This approach directly addresses the inconsistency introduced by the previous patch while maintaining the correct behavior of the queuing discipline management system. The solution aligns with established security practices for kernel memory management and reference counting, ensuring that all pointer operations are validated before resource manipulation occurs. This vulnerability demonstrates the importance of careful validation in kernel code where incorrect handling of edge cases can lead to subtle but serious memory management issues that may not manifest immediately but accumulate over time. The fix represents a defensive programming approach that prevents resource leaks by ensuring proper null pointer validation before reference counting operations, thereby maintaining system stability and preventing potential exploitation for denial of service attacks.
This vulnerability classification aligns with CWE-476 which describes NULL pointer dereference issues in software systems, and relates to ATT&CK technique T1499.004 which involves network denial of service through resource exhaustion. The issue specifically impacts the Linux kernel's networking subsystem and requires kernel-level patches to resolve, making it a critical vulnerability for system administrators to address promptly through kernel updates and security maintenance procedures.