CVE-2007-3722 in FreeBSD
Summary
by MITRE
The 4BSD process scheduler in the FreeBSD kernel performs scheduling based on CPU billing gathered from periodic process sampling ticks, which allows local users to cause a denial of service (CPU consumption) by performing voluntary nanosecond sleeps that result in the process not being active during a clock interrupt, as described in "Secretly Monopolizing the CPU Without Superuser Privileges."
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Analysis
by VulDB Data Team • 10/26/2017
The vulnerability described in CVE-2007-3722 resides within the FreeBSD kernel's 4BSD process scheduler implementation, which operates on a fundamental principle of CPU time accounting through periodic process sampling ticks. This scheduler mechanism relies on clock interrupts to gather CPU billing data from processes, creating a system where process activity is measured based on their presence during these periodic sampling intervals. The design flaw becomes apparent when considering how voluntary nanosecond sleeps are processed within this framework, as these brief sleep intervals can be manipulated to exploit the scheduler's timing assumptions. The vulnerability specifically targets the interaction between process sleep behavior and the periodic clock interrupt mechanism, creating a scenario where malicious processes can effectively monopolize CPU resources without requiring elevated privileges.
The technical exploitation occurs when a local user crafts voluntary sleep operations that strategically time their process execution to avoid participation during clock interrupt periods. This manipulation exploits the scheduler's assumption that processes will be active during sampling intervals, allowing the malicious process to consume CPU resources while remaining undetected by the scheduler's accounting mechanisms. The vulnerability stems from the scheduler's reliance on periodic sampling rather than continuous monitoring, creating a window where process activity can be artificially minimized during critical measurement periods. This design weakness enables a form of CPU resource manipulation that bypasses normal scheduling constraints, effectively allowing processes to consume CPU cycles without proper accounting, as detailed in the referenced "Secretly Monopolizing the CPU Without Superuser Privileges" analysis.
The operational impact of this vulnerability extends beyond simple resource consumption, as it represents a significant denial of service attack vector that can degrade system performance or completely monopolize CPU resources. Local users can leverage this vulnerability to create sustained CPU consumption patterns that may affect system responsiveness, impact other legitimate processes, or potentially cause system instability. The attack requires only local user privileges and can be executed through standard process management operations, making it particularly dangerous as it operates within normal system boundaries. The vulnerability demonstrates a fundamental flaw in the scheduler's time accounting methodology, where the timing of process execution relative to clock interrupts becomes a critical factor in resource allocation decisions. This creates a scenario where system resources can be consumed without proper authorization or monitoring, as the scheduler fails to accurately account for process activity during the exploitation window.
Mitigation strategies for CVE-2007-3722 must address the fundamental scheduling architecture flaw by implementing more robust CPU time accounting mechanisms that do not rely solely on periodic sampling intervals. System administrators should consider updating to patched FreeBSD versions that address this specific scheduler vulnerability, as the original implementation contained inherent design weaknesses that cannot be fully resolved through configuration changes alone. The fix typically involves modifying how the scheduler handles voluntary sleep operations and ensuring that CPU time accounting remains accurate regardless of process timing relative to clock interrupts. This vulnerability aligns with CWE-200, which addresses information exposure through improper resource management, and relates to ATT&CK technique T1496, which covers resource exhaustion attacks. Organizations should implement comprehensive monitoring of CPU usage patterns to detect anomalous behavior that might indicate exploitation attempts, while also ensuring that system updates are applied promptly to address known scheduling vulnerabilities. The remediation approach requires both architectural changes to the scheduler and operational procedures to detect and prevent exploitation attempts.