CVE-2021-39656 in Android
Summary
by MITRE • 12/15/2021
In __configfs_open_file of file.c, there is a possible use-after-free due to improper locking. This could lead to local escalation of privilege in the kernel with System execution privileges needed. User interaction is not needed for exploitation.Product: AndroidVersions: Android kernelAndroid ID: A-174049066References: Upstream kernel
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Analysis
by VulDB Data Team • 12/18/2021
The vulnerability identified as CVE-2021-39656 resides within the Android kernel's configfs implementation, specifically in the __configfs_open_file function located in file.c. This flaw represents a critical security weakness that demonstrates improper locking mechanisms during file operations within the kernel's configuration filesystem. The issue manifests as a use-after-free condition that occurs when multiple threads or processes attempt to access the same file resource concurrently without adequate synchronization. According to the upstream kernel references and Android ID A-174049066, this vulnerability affects Android kernel versions and represents a significant risk to system integrity.
The technical flaw stems from inadequate mutex or spinlock protection during file access operations within the configfs subsystem. When __configfs_open_file processes file requests, it fails to properly serialize access to shared resources, creating a race condition scenario where one thread might free a memory object while another thread attempts to reference it. This improper locking allows for a scenario where a freed kernel memory structure is still being accessed, leading to undefined behavior that can be exploited by malicious code. The vulnerability falls under CWE-415 which specifically addresses double free conditions and CWE-362 which covers race conditions in concurrent programming environments. The flaw is particularly dangerous because it occurs within kernel space where the attacker can leverage the use-after-free to execute arbitrary code with the highest privileges available to the kernel.
The operational impact of this vulnerability extends to local privilege escalation attacks, where an unprivileged user can potentially elevate their privileges to system level execution rights. This occurs because the use-after-free condition allows for memory corruption that can be manipulated to redirect execution flow or inject malicious code into kernel memory spaces. The exploitation requires only local system execution privileges, meaning that any user with access to the system can potentially leverage this vulnerability. The attack vector does not require user interaction, making it particularly dangerous as it can be exploited automatically without any human intervention. This characteristic aligns with ATT&CK technique T1068 which covers local privilege escalation through kernel exploits, and T1059 which covers execution through kernel modules or direct memory manipulation.
Mitigation strategies for CVE-2021-39656 should focus on implementing proper locking mechanisms within the __configfs_open_file function to ensure thread safety during concurrent file access operations. The fix requires adding appropriate mutex or spinlock acquisition before accessing shared resources and ensuring proper release of these locks after operations complete. Android kernel developers should also consider implementing additional validation checks to prevent premature memory deallocation during active file operations. Security patches should address the root cause by ensuring that file descriptors and associated kernel structures remain valid throughout the duration of access operations. Regular kernel updates and security audits should be performed to identify similar race condition vulnerabilities in other subsystems. Organizations should also implement monitoring solutions to detect potential exploitation attempts and maintain comprehensive incident response procedures for kernel-level security breaches. The vulnerability demonstrates the critical importance of proper synchronization in kernel code and highlights the need for thorough security testing of concurrent programming constructs within operating system components.