CVE-2016-6516 in Linux
Summary
by MITRE
Race condition in the ioctl_file_dedupe_range function in fs/ioctl.c in the Linux kernel through 4.7 allows local users to cause a denial of service (heap-based buffer overflow) or possibly gain privileges by changing a certain count value, aka a "double fetch" vulnerability.
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Analysis
by VulDB Data Team • 09/13/2022
The vulnerability identified as CVE-2016-6516 represents a critical race condition within the Linux kernel's file system operations, specifically affecting the ioctl_file_dedupe_range function located in fs/ioctl.c. This flaw exists in kernel versions through 4.7 and demonstrates a classic double fetch vulnerability pattern that can be exploited by local attackers to achieve either denial of service or privilege escalation. The vulnerability stems from improper synchronization mechanisms during file deduplication operations, where concurrent access to shared data structures creates opportunities for malicious manipulation of memory contents.
The technical implementation of this vulnerability involves a heap-based buffer overflow that occurs when the kernel processes file deduplication requests through ioctl commands. During the execution of ioctl_file_dedupe_range, the kernel performs a double fetch operation where the same data is accessed twice without proper validation or synchronization between fetches. This race condition allows an attacker to manipulate the count parameter value in such a way that subsequent memory operations exceed allocated buffer boundaries, leading to heap corruption. The flaw is particularly dangerous because it operates at kernel level where memory corruption can result in arbitrary code execution, making it a prime target for privilege escalation attacks.
The operational impact of CVE-2016-6516 extends beyond simple denial of service scenarios to encompass potential system compromise through privilege escalation. Local attackers with basic user privileges can exploit this vulnerability to gain elevated permissions, potentially leading to complete system control. The heap-based buffer overflow creates memory corruption that can be leveraged to execute malicious code within kernel space, bypassing standard user-mode protections. This vulnerability affects systems running Linux kernel versions up to 4.7, making it particularly concerning for organizations that have not yet updated their kernel versions.
From a cybersecurity perspective, this vulnerability aligns with CWE-362, which describes race conditions in concurrent programming, and maps to ATT&CK technique T1068, which covers "Exploitation for Privilege Escalation." The double fetch pattern exploited in this vulnerability is a well-documented class of memory corruption flaws that have been consistently identified in kernel codebases. The vulnerability's exploitation requires local access but provides a pathway for attackers to escalate privileges, making it a significant concern for system administrators who must maintain up-to-date kernel versions. Organizations should prioritize patching this vulnerability through kernel updates, as the risk of exploitation increases with the complexity of file system operations and the prevalence of deduplication features in modern storage systems.
Mitigation strategies for CVE-2016-6516 primarily focus on kernel version updates to versions beyond 4.7 where the race condition has been addressed through proper synchronization mechanisms. System administrators should implement comprehensive patch management processes to ensure all kernel components are current with security fixes. Additional protective measures include monitoring for suspicious ioctl operations and implementing kernel hardening techniques such as stack canaries and address space layout randomization to complicate exploitation attempts. The vulnerability also highlights the importance of rigorous code review processes for kernel functions that handle file system operations, particularly those involving concurrent access patterns and memory management operations.