CVE-2009-4895 in Linux
Summary
by MITRE
Race condition in the tty_fasync function in drivers/char/tty_io.c in the Linux kernel before 2.6.32.6 allows local users to cause a denial of service (NULL pointer dereference and system crash) or possibly have unspecified other impact via unknown vectors, related to the put_tty_queue and __f_setown functions. NOTE: the vulnerability was addressed in a different way in 2.6.32.9.
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Analysis
by VulDB Data Team • 09/24/2021
The vulnerability identified as CVE-2009-4895 represents a critical race condition within the Linux kernel's terminal I/O subsystem, specifically affecting the tty_fasync function located in drivers/char/tty_io.c. This flaw exists in kernel versions prior to 2.6.32.6 and demonstrates a fundamental concurrency issue that can be exploited by local attackers to compromise system stability. The race condition occurs during the asynchronous notification handling mechanism for terminal devices, creating a window where multiple threads or processes can simultaneously access shared data structures without proper synchronization, leading to unpredictable behavior and potential system failure.
The technical implementation of this vulnerability stems from improper handling of file descriptor ownership and asynchronous notification registration within the terminal subsystem. When the put_tty_queue and __f_setown functions interact with the tty_fasync function, a race condition emerges that allows for a NULL pointer dereference scenario. This occurs because the system fails to properly validate the state of terminal queue structures during concurrent access operations, particularly when multiple processes attempt to modify or access the same terminal device's asynchronous notification settings simultaneously. The flaw operates at the kernel level where the race condition manifests during the transition period between setting ownership and processing asynchronous notifications, creating a temporal gap that malicious code can exploit through carefully crafted system calls or concurrent process execution patterns.
The operational impact of this vulnerability extends beyond simple denial of service to potentially enable more severe system compromise depending on the exploitation vector and system configuration. Local attackers can trigger system crashes through NULL pointer dereference conditions that cause kernel panics and immediate system termination, effectively rendering the affected system unusable until manual reboot occurs. However, the description notes that the vulnerability may also enable unspecified other impacts, suggesting potential for privilege escalation or information disclosure scenarios that could arise from the underlying race condition's influence on kernel memory management and process scheduling. The vulnerability's exploitation requires local access and system privileges, making it less severe than remote exploits but still highly concerning for systems where local user access cannot be fully controlled.
Mitigation strategies for CVE-2009-4895 focus on immediate kernel version upgrades to 2.6.32.9 or later, which contains the specific patches addressing the race condition through improved synchronization mechanisms and proper validation of terminal queue states. System administrators should prioritize updating kernel versions across all affected systems, particularly those running older kernel versions where this vulnerability remains present. Additional protective measures include implementing proper access controls to limit local user privileges, monitoring for unusual system crashes or kernel panic events that might indicate exploitation attempts, and maintaining comprehensive system logging to track potential race condition scenarios. The vulnerability aligns with CWE-362, which catalogs race conditions in software systems, and represents a classic example of improper synchronization leading to memory corruption and system instability. Organizations should also consider implementing kernel hardening measures such as stack canaries, address space layout randomization, and other security enhancements that can further reduce the attack surface and potential impact of similar concurrency flaws.