CVE-2016-6197 in Linux
Summary
by MITRE
fs/overlayfs/dir.c in the OverlayFS filesystem implementation in the Linux kernel before 4.6 does not properly verify the upper dentry before proceeding with unlink and rename system-call processing, which allows local users to cause a denial of service (system crash) via a rename system call that specifies a self-hardlink.
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Analysis
by VulDB Data Team • 09/13/2022
The vulnerability identified as CVE-2016-6197 resides within the OverlayFS filesystem implementation in Linux kernel versions prior to 4.6. This flaw manifests in the fs/overlayfs/dir.c file where the system fails to adequately validate the upper dentry during unlink and rename operations. The issue represents a critical security gap that enables local attackers to exploit the kernel's handling of filesystem operations. OverlayFS is a union filesystem that allows multiple filesystems to be mounted together, creating a single unified filesystem view, and it became widely adopted for its flexibility in containerized environments and various Linux distributions.
The technical exploitation of this vulnerability occurs when a local user executes a rename system call that creates a self-hardlink within the OverlayFS filesystem. The kernel's insufficient verification of the upper dentry during these operations results in a race condition or improper state handling that leads to a kernel crash. This occurs because the overlayfs implementation does not properly check whether the target of a rename operation references itself, creating a circular reference that the kernel cannot handle gracefully. The flaw essentially allows an attacker to manipulate the filesystem metadata in such a way that subsequent kernel operations fail catastrophically, resulting in system instability.
The operational impact of this vulnerability extends beyond simple denial of service, as it can lead to complete system crashes and require manual intervention to restore normal operation. When exploited successfully, the vulnerability causes the kernel to panic and may require a system reboot to recover from the crash state. This type of vulnerability is particularly concerning in production environments where system stability is paramount, as it can be leveraged to disrupt services or potentially create conditions that might allow further exploitation. The local privilege requirement means that an attacker must already have access to the system to exploit this vulnerability, but the potential for system-wide disruption remains significant.
Mitigation strategies for CVE-2016-6197 primarily involve upgrading to Linux kernel version 4.6 or later, where the vulnerability has been patched through improved validation of upper dentry references in the overlayfs implementation. System administrators should also consider implementing additional monitoring to detect unusual filesystem operations that might indicate exploitation attempts. The vulnerability aligns with CWE-119, which addresses improper access to memory locations, and relates to ATT&CK technique T1059, specifically the use of system calls to manipulate filesystem state. Organizations should prioritize patch management to ensure all systems running affected kernel versions receive updates promptly, as the vulnerability can be exploited without elevated privileges and requires no special tools beyond standard system utilities.