CVE-2016-8660 in Linux
Summary
by MITRE
The XFS subsystem in the Linux kernel through 4.8.2 allows local users to cause a denial of service (fdatasync failure and system hang) by using the vfs syscall group in the trinity program, related to a "page lock order bug in the XFS seek hole/data implementation."
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Analysis
by VulDB Data Team • 05/11/2019
The vulnerability identified as CVE-2016-8660 represents a critical flaw within the XFS filesystem implementation of the Linux kernel version 4.8.2 and earlier. This issue specifically targets the XFS subsystem's handling of file operations, particularly those involving the fdatasync system call and the trinity program's use of the vfs syscall group. The vulnerability manifests as a denial of service condition that can result in complete system hangs, making it particularly dangerous in production environments where system availability is paramount.
The technical root cause of this vulnerability lies in a page lock order bug within the XFS seek hole/data implementation. This type of bug typically occurs when the kernel's locking mechanism encounters an improper ordering of page locks during concurrent file operations, creating a potential deadlock scenario. The flaw is specifically triggered when the trinity program, a stress testing tool designed to identify kernel-level bugs, utilizes the vfs syscall group to perform operations that involve seeking through file holes and data blocks. The page lock order bug creates a situation where multiple locks are acquired in an inconsistent order, leading to a circular dependency that results in system-wide locking and eventual system hang.
The operational impact of CVE-2016-8660 extends beyond simple denial of service, as it can effectively render a system unusable until manual intervention occurs. Attackers with local access can exploit this vulnerability to cause system-wide hangs that require system reboot to resolve, creating significant availability issues for services running on affected systems. This vulnerability is particularly concerning because it affects the core filesystem operations that are fundamental to system operation, meaning that any process attempting to perform file synchronization operations could trigger the bug. The vulnerability's classification aligns with CWE-121, which deals with stack-based buffer overflow conditions, and more specifically with CWE-123, which addresses the issue of lock ordering problems in concurrent systems. From an attack framework perspective, this vulnerability maps to the ATT&CK technique T1499.004, which involves network denial of service attacks, though in this case the attack vector is local and involves system-level resource exhaustion.
The exploitation of this vulnerability requires local system access and the ability to execute the trinity stress testing program, which makes it less likely to be exploited remotely but still poses a significant risk in environments where local access is possible. The vulnerability demonstrates the complexity of modern kernel development where seemingly minor implementation details in filesystem code can lead to catastrophic system failures. Organizations running Linux systems with XFS filesystems should prioritize patching to address this vulnerability, as it represents a fundamental flaw in the kernel's locking mechanisms that could be exploited by malicious users with local access to cause system-wide outages. The fix for this vulnerability involved correcting the page lock ordering in the XFS seek hole/data implementation, ensuring that locks are acquired in a consistent order to prevent the deadlock condition that leads to system hangs.