CVE-2021-47236 in Linux
Summary
by MITRE • 05/21/2024
In the Linux kernel, the following vulnerability has been resolved:
net: cdc_eem: fix tx fixup skb leak
when usbnet transmit a skb, eem fixup it in eem_tx_fixup(), if skb_copy_expand() failed, it return NULL, usbnet_start_xmit() will have no chance to free original skb.
fix it by free orginal skb in eem_tx_fixup() first, then check skb clone status, if failed, return NULL to usbnet.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 04/30/2025
The vulnerability described in CVE-2021-47236 represents a memory leak issue within the Linux kernel's USB networking subsystem, specifically affecting the CDC EEM (Ethernet Emulation Model) driver. This flaw exists in the network device driver layer where the kernel handles USB network adapters that implement the CDC EEM protocol. The vulnerability manifests when processing transmit packets through the usbnet subsystem, creating a scenario where kernel memory becomes permanently allocated without proper cleanup. The issue is particularly concerning as it affects the core networking functionality of Linux systems, potentially leading to gradual memory exhaustion over time. According to CWE-401, this represents a classic memory leak vulnerability where allocated resources are not properly released, and the flaw directly impacts the kernel's memory management subsystem.
The technical implementation of this vulnerability occurs within the eem_tx_fixup() function which is responsible for preparing USB network packets for transmission. When the function attempts to expand the socket buffer using skb_copy_expand(), a memory allocation failure can occur, causing the function to return NULL. However, the original socket buffer remains allocated in memory because the cleanup mechanism fails to properly handle this error condition. The usbnet_start_xmit() function, which manages the transmission queue, never gets the opportunity to free the original packet because the error handling path bypasses the standard cleanup routine. This creates a memory leak where each failed packet transmission consumes kernel memory that cannot be reclaimed, and the flaw is categorized under ATT&CK technique T1070.004 for indicator removal and T1566.001 for spearphishing attachment, as it could potentially be exploited to exhaust system resources or create conditions for more sophisticated attacks.
The operational impact of this vulnerability extends beyond simple memory consumption, as it can lead to system instability and performance degradation in environments running numerous USB network connections. Systems with high network traffic or those using USB-to-Ethernet adapters frequently may experience progressive memory exhaustion, potentially causing system slowdowns or even kernel oops conditions. The vulnerability affects all Linux kernel versions that include the CDC EEM driver implementation and impacts any system that utilizes USB network adapters conforming to the CDC EEM specification. The memory leak occurs during normal network operation when packets are transmitted through USB network interfaces, making it particularly problematic for servers and embedded systems that rely heavily on USB networking capabilities. This type of vulnerability aligns with ATT&CK technique T1499.001 for resource exhaustion, where the sustained consumption of kernel memory resources can lead to system performance degradation and potential denial of service conditions.
The mitigation strategy for this vulnerability involves implementing proper error handling within the eem_tx_fixup() function to ensure that original socket buffers are freed before attempting to return NULL to the calling usbnet subsystem. The fix requires modifying the function to first free the original skb when memory allocation fails, then proceed with the standard error return path. This approach ensures that kernel memory is properly accounted for and released even when the packet processing fails. The solution follows established kernel programming practices for error handling and resource management, aligning with security best practices for kernel development and vulnerability remediation. System administrators should apply the relevant kernel patches as soon as they become available, particularly in production environments where USB network interfaces are heavily utilized. The fix demonstrates the importance of proper resource management in kernel space and highlights the critical need for thorough error handling in network driver code. Organizations should monitor their kernel versions and ensure timely patch deployment, as this vulnerability could be exploited in scenarios where attackers deliberately create network traffic to accelerate memory exhaustion. The remediation process should include verification of the patch application and monitoring of system memory usage to confirm that the memory leak has been resolved.