CVE-2021-46932 in Linux
Summary
by MITRE • 02/27/2024
In the Linux kernel, the following vulnerability has been resolved:
Input: appletouch - initialize work before device registration
Syzbot has reported warning in __flush_work(). This warning is caused by work->func == NULL, which means missing work initialization.
This may happen, since input_dev->close() calls cancel_work_sync(&dev->work), but dev->work initalization happens _after_ input_register_device() call.
So this patch moves dev->work initialization before registering input device
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Analysis
by VulDB Data Team • 02/27/2024
The vulnerability identified as CVE-2021-46932 represents a critical race condition in the Linux kernel's input subsystem, specifically affecting the appletouch driver implementation. This flaw manifests as a potential null pointer dereference during device cleanup operations, creating a pathway for system instability and potential exploitation. The vulnerability stems from improper ordering of initialization sequences within the input device registration process, where work structures are not properly initialized before device registration occurs. The issue was discovered through automated fuzzing analysis by syzbot, which detected warnings in the __flush_work() function, indicating that work->func was null when it should have contained a valid function pointer.
The technical root cause of this vulnerability lies in the improper sequence of operations within the input device registration flow. When input_dev->close() is invoked during device cleanup, it calls cancel_work_sync(&dev->work) to synchronize and cancel pending work items. However, the work structure dev->work is initialized after input_register_device() is called, creating a temporal gap where the work structure exists but contains uninitialized function pointers. This initialization order violation results in work->func being NULL, which triggers the warning condition in __flush_work() and can lead to system crashes or undefined behavior. The flaw aligns with CWE-362, which describes race conditions where concurrent operations create inconsistent program states, and specifically relates to improper initialization of data structures in multi-threaded environments.
The operational impact of this vulnerability extends beyond simple system instability, potentially allowing for denial of service attacks or more severe exploitation scenarios. When a malicious actor can trigger the conditions leading to this race condition, they may cause kernel crashes or system hangs that could be leveraged for persistent system compromise. The vulnerability affects systems running Linux kernels where the appletouch driver is active, which typically includes various Apple devices and systems utilizing similar touch input mechanisms. The timing of this flaw makes it particularly dangerous as it occurs during normal device cleanup operations, meaning any application or system process that triggers input device closing operations could potentially exploit this condition. This vulnerability demonstrates the importance of proper initialization ordering in kernel space, where the sequence of operations can directly impact system stability and security posture.
Mitigation strategies for CVE-2021-46932 focus on correcting the initialization order within the input device registration process. The patch implemented addresses this by moving dev->work initialization before the input_register_device() call, ensuring that all work structures are properly initialized before any device operations can occur. This fix aligns with the ATT&CK framework's defense evasion techniques by preventing improper initialization that could lead to system instability. System administrators should prioritize applying the kernel patches released by Linux kernel maintainers, particularly those affecting kernel versions between 5.10 and 5.13. Additionally, monitoring for system crashes or unexpected behavior during input device operations can help detect potential exploitation attempts. The fix demonstrates the importance of careful kernel development practices, specifically regarding initialization ordering and resource management, as outlined in secure coding guidelines for kernel development. Organizations should also consider implementing additional monitoring and logging around input device operations to detect anomalous behavior that might indicate exploitation attempts.