CVE-2021-47232 in Linux
Summary
by MITRE • 05/21/2024
In the Linux kernel, the following vulnerability has been resolved:
can: j1939: fix Use-after-Free, hold skb ref while in use
This patch fixes a Use-after-Free found by the syzbot.
The problem is that a skb is taken from the per-session skb queue, without incrementing the ref count. This leads to a Use-after-Free if the skb is taken concurrently from the session queue due to a CTS.
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Analysis
by VulDB Data Team • 05/21/2024
The vulnerability CVE-2021-47232 represents a critical use-after-free condition within the Linux kernel's CAN (Controller Area Network) subsystem, specifically in the J1939 protocol implementation. This flaw exists in the kernel's networking stack where the CAN subsystem handles automotive network communications, making it particularly concerning for automotive and industrial control systems. The vulnerability was identified through automated fuzzing by syzbot, which detected the race condition during concurrent access to socket buffer (skb) management within the J1939 protocol handler. The issue manifests when the kernel processes CAN frames according to the J1939 standard, which is commonly used in heavy-duty vehicles and industrial machinery for real-time communication between electronic control units.
The technical root cause of this vulnerability stems from improper reference counting of socket buffers within the per-session queue management system. When the kernel retrieves an skb from a session queue for processing, it fails to increment the reference count before releasing the queue lock. This creates a race condition where another concurrent process can simultaneously access and free the same skb while the original process is still using it. The specific scenario occurs during CTS (Clear To Send) handling, where the kernel must manage flow control for CAN messages. The lack of proper reference counting means that when one thread processes an skb and another thread frees it concurrently, the first thread attempts to access memory that has already been deallocated, resulting in undefined behavior and potential system instability.
The operational impact of CVE-2021-47232 extends beyond simple system crashes, as it can potentially enable privilege escalation or denial-of-service conditions in systems relying on CAN network communications. Automotive systems, industrial automation equipment, and embedded devices that utilize the J1939 protocol for vehicle communication are particularly vulnerable, as these systems often operate in safety-critical environments where system reliability is paramount. The vulnerability affects any Linux kernel version that includes the CAN subsystem with J1939 support, making it widespread across automotive and industrial deployments. Attackers could exploit this condition to cause system panics, corrupt memory, or potentially execute arbitrary code with kernel privileges, especially in environments where CAN traffic processing is critical for system operation.
Mitigation strategies for CVE-2021-47232 require immediate kernel updates to the patched versions that implement proper reference counting mechanisms for socket buffers during concurrent access scenarios. System administrators should prioritize patching all affected Linux systems, particularly those running automotive or industrial control applications that utilize CAN J1939 communications. The fix implements a proper reference count increment before releasing the queue lock, ensuring that the skb remains allocated for the duration of its use. Additional defensive measures include monitoring for unusual CAN traffic patterns, implementing network segmentation to limit exposure, and ensuring proper access controls on systems handling critical CAN communications. Organizations should also conduct vulnerability assessments to identify systems running affected kernel versions and establish monitoring protocols for detecting potential exploitation attempts, as the vulnerability can be leveraged to cause system instability in critical infrastructure environments where CAN communications are essential for operation.
This vulnerability aligns with CWE-415: Double Free and CWE-416: Use After Free categories from the Common Weakness Enumeration catalog, representing a classic race condition that leads to memory safety violations. The attack pattern follows ATT&CK technique T1059.007: Command and Scripting Interpreter: Python, though more accurately described as a kernel-level memory corruption technique that can lead to privilege escalation. The vulnerability demonstrates the importance of proper resource management in kernel space and highlights the critical need for thorough testing of concurrent access scenarios in network protocol implementations.