CVE-2021-47239 in Linux
Summary
by MITRE • 05/21/2024
In the Linux kernel, the following vulnerability has been resolved:
net: usb: fix possible use-after-free in smsc75xx_bind
The commit 46a8b29c6306 ("net: usb: fix memory leak in smsc75xx_bind") fails to clean up the work scheduled in smsc75xx_reset-> smsc75xx_set_multicast, which leads to use-after-free if the work is scheduled to start after the deallocation. In addition, this patch also removes a dangling pointer - dev->data[0].
This patch calls cancel_work_sync to cancel the scheduled work and set the dangling pointer to NULL.
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Analysis
by VulDB Data Team • 12/30/2024
The vulnerability identified as CVE-2021-47239 represents a critical use-after-free condition within the Linux kernel's USB networking driver subsystem, specifically affecting the smsc75xx driver. This flaw exists in the network interface driver responsible for managing SMSC USB Ethernet adapters, which are commonly found in various networking devices including USB-to-Ethernet adapters and embedded systems. The vulnerability stems from improper memory management during the driver binding process, where the kernel fails to properly handle cleanup operations for scheduled work items, creating a scenario where freed memory can be accessed after deallocation.
The technical root cause of this vulnerability lies in the implementation of the smsc75xx_bind function within the USB networking driver framework. The original fix for a memory leak in commit 46a8b29c6306 inadvertently introduced this use-after-free condition by failing to properly cancel work items that were scheduled during the driver initialization sequence. Specifically, when the smsc75xx_reset function calls smsc75xx_set_multicast, it schedules work items that are not properly canceled before the driver structure is deallocated. This creates a race condition where the work item may execute after the memory containing the device structure has been freed, leading to memory corruption and potential arbitrary code execution.
The operational impact of this vulnerability extends beyond simple memory corruption, as it can be exploited to achieve privilege escalation or system compromise within the kernel space. Attackers who can trigger the specific sequence leading to the use-after-free condition can potentially execute arbitrary code with kernel privileges, undermining the security model of the Linux operating system. The vulnerability is particularly concerning because it affects the USB networking subsystem, which is widely used across various Linux distributions and embedded systems, making it a potentially widespread security risk. According to CWE classification, this vulnerability maps to CWE-416: Use After Free, which is a well-known class of memory safety issues that have historically led to numerous security exploits.
The patch addressing this vulnerability implements proper cleanup procedures by invoking cancel_work_sync to ensure all scheduled work items are properly canceled before memory deallocation occurs. This approach aligns with established best practices for kernel memory management and follows the principles outlined in the Linux kernel security guidelines. Additionally, the patch addresses a dangling pointer issue by setting dev->data[0] to NULL, preventing potential access to freed memory locations. This remediation technique is consistent with the ATT&CK framework's approach to mitigating kernel-level vulnerabilities, where proper resource management and memory cleanup are essential defensive measures. The solution demonstrates the importance of comprehensive testing for security patches, as the initial fix for one vulnerability inadvertently created another, highlighting the complexity of kernel security maintenance and the need for thorough validation of security fixes.