CVE-2011-2491 in Linux
Summary
by MITRE
The Network Lock Manager (NLM) protocol implementation in the NFS client functionality in the Linux kernel before 3.0 allows local users to cause a denial of service (system hang) via a LOCK_UN flock system call.
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Analysis
by VulDB Data Team • 12/29/2021
The Network Lock Manager protocol within the Linux kernel's Network File System client implementation presents a critical vulnerability that enables local users to trigger system hangs through improper handling of LOCK_UN flock system calls. This vulnerability exists in kernel versions prior to 30 and affects the fundamental locking mechanisms that govern file access in distributed file systems. The NLM protocol operates as a crucial component for managing file locks across networked systems, coordinating access to shared resources between multiple clients and servers. When a local user executes a LOCK_UN system call against an NFS-mounted filesystem, the kernel's processing of this request fails to properly handle the unlock operation, leading to a state where the system becomes unresponsive and enters a condition that requires manual intervention to resolve.
The technical flaw manifests in the kernel's NLM implementation where the LOCK_UN system call does not properly validate or process the unlock request when dealing with certain file locking scenarios in NFS environments. This occurs because the kernel's lock management subsystem fails to properly release resources or update internal lock state information when processing unlock operations, resulting in a deadlock condition that causes the system to hang indefinitely. The vulnerability specifically impacts the kernel's file locking subsystem and operates at the kernel level where the NFS client functionality interfaces with the local file system locking mechanisms. This represents a classic case of improper resource handling that can be exploited by local users without requiring special privileges or network access, making it particularly dangerous in multi-user environments where such users may have legitimate access to the system.
The operational impact of this vulnerability extends beyond simple denial of service, as system hangs can result in complete service disruption and potential data loss if the system remains unresponsive for extended periods. Local users can exploit this weakness to disrupt normal system operations, potentially affecting other running services and applications that depend on file system access. The vulnerability particularly impacts servers and workstations that utilize NFS for file sharing, as the locking mechanism is essential for maintaining data consistency across networked file systems. When the system becomes unresponsive due to this vulnerability, administrators must manually intervene to restart affected services or reboot the system, resulting in unplanned downtime and potential business disruption. The exploitability of this vulnerability through local access means that it can be triggered by any user with access to the system, regardless of their administrative privileges, making it a significant security concern for system administrators.
Mitigation strategies for this vulnerability involve upgrading the Linux kernel to version 3.0 or later where the NLM implementation has been corrected to properly handle LOCK_UN system calls and prevent the deadlock conditions that cause system hangs. System administrators should also implement monitoring solutions to detect unusual lock behavior or system hang conditions that may indicate exploitation attempts. Additionally, organizations should consider restricting local user access to NFS-mounted filesystems where possible and implementing proper access controls to minimize the attack surface. The vulnerability aligns with CWE-121 which describes improper restriction of operations within a fixed storage buffer, and relates to ATT&CK technique T1499.004 for endpoint denial of service through resource exhaustion. Regular kernel updates and patch management procedures should be implemented to ensure that systems remain protected against similar vulnerabilities in the NLM protocol implementation, as this type of kernel-level flaw can have cascading effects on system stability and availability.