CVE-2017-8925 in Linux
Summary
by MITRE
The omninet_open function in drivers/usb/serial/omninet.c in the Linux kernel before 4.10.4 allows local users to cause a denial of service (tty exhaustion) by leveraging reference count mishandling.
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Analysis
by VulDB Data Team • 12/06/2022
The vulnerability identified as CVE-2017-8925 resides within the Linux kernel's USB serial driver implementation, specifically in the omninet_open function located in drivers/usb/serial/omninet.c. This flaw represents a critical issue that affects systems running Linux kernel versions prior to 4.10.4, creating a significant security risk for local users who can exploit it to trigger a denial of service condition. The vulnerability stems from improper reference count management within the kernel's USB serial subsystem, which governs how the kernel handles communication with USB serial devices through the tty framework.
The technical flaw manifests through reference count mishandling that occurs during the opening of USB serial devices via the omninet driver. When a local user attempts to open a USB serial device using this driver, the kernel fails to properly increment or decrement reference counts associated with the tty structure. This improper management leads to a situation where the kernel's tty subsystem becomes exhausted, preventing further legitimate device access and effectively rendering the serial communication capabilities unavailable. The vulnerability operates at the kernel level and leverages the inherent trust model of local users, making it particularly dangerous as it requires no elevated privileges to exploit.
The operational impact of this vulnerability extends beyond simple service disruption to potentially compromise system stability and availability of serial communication services. When exploited, the vulnerability causes tty exhaustion which affects not just the specific omninet device but can potentially impact the entire system's serial communication capabilities. This denial of service condition can affect critical system functions that depend on serial communication, including system logging, hardware monitoring, and device management operations. The vulnerability is particularly concerning in embedded systems, industrial control environments, or any infrastructure where reliable serial communication is essential for system operation, as the impact can cascade to affect broader system functionality.
The flaw aligns with CWE-475, which describes improper handling of reference counts, and demonstrates characteristics consistent with the ATT&CK technique T1499.004 for network denial of service. The vulnerability represents a classic example of resource exhaustion through improper reference counting mechanisms, where the kernel's internal accounting becomes corrupted, leading to the depletion of available tty structures. System administrators and security professionals should prioritize patching affected systems to ensure that kernel versions 4.10.4 and later are deployed, as these releases contain the necessary fixes to properly manage reference counts within the omninet driver. Additionally, monitoring for unusual tty allocation patterns or system resource exhaustion events should be implemented as part of defensive measures, particularly in environments where USB serial devices are actively utilized. The remediation process requires updating the kernel to a patched version while maintaining careful attention to ensure that the updated system continues to support required USB serial device functionality without introducing new compatibility issues.