CVE-2011-4621 in Linux
Summary
by MITRE
The Linux kernel before 2.6.37 does not properly implement a certain clock-update optimization, which allows local users to cause a denial of service (system hang) via an application that executes code in a loop.
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Analysis
by VulDB Data Team • 03/21/2021
The vulnerability identified as CVE-2011-4621 represents a critical flaw in the Linux kernel's time management subsystem that affects versions prior to 2.6.37. This issue stems from an improper implementation of a clock-update optimization mechanism that was intended to improve system performance by reducing unnecessary time updates. The flaw manifests when malicious applications exploit this optimization to create a condition where the system's clock handling becomes unresponsive, ultimately leading to a complete system hang. The vulnerability specifically targets the kernel's handling of time-related operations and demonstrates how performance optimizations can introduce serious stability risks when not properly implemented.
The technical root cause of this vulnerability lies in the kernel's timer subsystem where the clock-update optimization fails to properly account for certain edge cases in time synchronization. When an application continuously executes code that triggers the problematic time update path, it causes the kernel to enter a state where it cannot properly process time-related interrupts or update the system clock. This creates a deadlock condition where the system becomes unresponsive to user input and normal processing operations. The issue is particularly insidious because it requires only a local user with standard privileges to exploit, making it accessible through any application that can repeatedly execute the problematic code path. This vulnerability falls under the CWE-362 category of Concurrent Execution using Shared Resource with Unprotected Critical Section, as it involves improper synchronization in kernel time management code.
The operational impact of CVE-2011-4621 extends beyond simple denial of service to potentially compromise entire system availability. A local attacker can easily cause a system hang that requires manual intervention or system reboot to resolve, effectively creating a persistent denial of service condition. The vulnerability is particularly dangerous in server environments where system uptime is critical, as it can be exploited to disrupt services without requiring elevated privileges or special access. This makes it a preferred attack vector for adversaries seeking to cause disruption while maintaining low operational complexity. The vulnerability also aligns with ATT&CK technique T1499.004 which covers "Endpoint Denial of Service" and demonstrates how kernel-level flaws can be leveraged to create system-wide availability issues that bypass traditional security controls.
Mitigation strategies for CVE-2011-4621 primarily focus on kernel version updates to 2.6.37 or later, where the problematic clock-update optimization has been properly implemented. System administrators should prioritize patching affected systems and verify that the updated kernel properly implements the time management subsystem without the vulnerable optimization. Additionally, monitoring systems should be configured to detect unusual patterns in system time updates or potential deadlock conditions that could indicate exploitation attempts. The vulnerability highlights the importance of thorough testing of performance optimizations in kernel code and demonstrates why security reviews should include careful examination of time-sensitive operations. Organizations should also implement proper system hardening measures including limiting local user privileges where possible and monitoring for unusual system behavior that could indicate exploitation attempts. The fix implemented in kernel version 2.6.37 addresses the synchronization issue in the timer subsystem and ensures that clock updates properly handle all edge cases without creating deadlock conditions that could lead to system hangs.