CVE-2015-4178 in Linux
Summary
by MITRE
The fs_pin implementation in the Linux kernel before 4.0.5 does not ensure the internal consistency of a certain list data structure, which allows local users to cause a denial of service (system crash) by leveraging user-namespace root access for an MNT_DETACH umount2 system call, related to fs/fs_pin.c and include/linux/fs_pin.h.
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Analysis
by VulDB Data Team • 07/27/2022
The vulnerability identified as CVE-2015-4178 resides within the Linux kernel's filesystem pinning mechanism, specifically in the fs_pin implementation that manages reference counting for mounted filesystems. This flaw affects kernel versions prior to 4.0.5 and represents a critical consistency issue that can be exploited to trigger system crashes. The vulnerability manifests when a user with root privileges within a user namespace attempts to perform an MNT_DETACH umount2 system call, which should normally be a safe operation for detaching filesystem mounts. The underlying problem stems from improper handling of internal data structures that track filesystem references, creating a condition where the list management logic fails to maintain proper state consistency.
The technical implementation of this vulnerability involves a race condition and data structure corruption within the kernel's filesystem subsystem. When the MNT_DETACH flag is used in conjunction with the umount2 system call, the kernel's fs_pin.c module fails to properly validate or update the internal list structure that tracks filesystem references. This inconsistency allows malicious code to manipulate the reference counting mechanism in such a way that subsequent operations on the filesystem data structures cause kernel panics or system crashes. The vulnerability specifically targets the interaction between user namespaces and mount operations, where the privilege escalation within a user namespace provides the attacker with sufficient capabilities to manipulate kernel memory structures.
The operational impact of CVE-2015-4178 extends beyond simple denial of service to potentially compromise system stability and availability. An attacker with root access within a user namespace can reliably crash the kernel by triggering the specific sequence of operations that exploit the fs_pin consistency issue. This vulnerability directly maps to CWE-119, which describes weaknesses in memory management, and can be categorized under ATT&CK technique T1499.200 for endpoint denial of service. The exploit requires minimal privileges within a user namespace but can result in complete system instability, making it particularly dangerous in multi-tenant environments or systems where user namespaces are utilized for containerization. The vulnerability affects systems running kernel versions before 4.0.5 and can be leveraged by attackers to disrupt services or potentially escalate privileges further by causing system reboots that might be exploited in certain scenarios.
The mitigation strategy for this vulnerability requires immediate kernel updates to version 4.0.5 or later, where the fs_pin implementation has been corrected to maintain proper list consistency. System administrators should prioritize patching affected systems, particularly those running containers or virtualized environments where user namespaces are commonly used. Additional protective measures include implementing strict user namespace restrictions and monitoring for unusual mount/unmount patterns. The fix addresses the root cause by ensuring proper locking mechanisms and validation checks in the filesystem pinning code, preventing the corruption of internal data structures that occurred in vulnerable versions. Organizations should also consider implementing kernel lockdown features and privileged access controls to limit the potential impact of any remaining vulnerabilities in their systems.