CVE-2017-2636 in Linux
Summary
by MITRE
Race condition in drivers/tty/n_hdlc.c in the Linux kernel through 4.10.1 allows local users to gain privileges or cause a denial of service (double free) by setting the HDLC line discipline.
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Analysis
by VulDB Data Team • 09/04/2020
The vulnerability identified as CVE-2017-2636 represents a critical race condition flaw within the Linux kernel's tty subsystem, specifically in the drivers/tty/n_hdlc.c file. This issue affects kernel versions through 4.10.1 and exposes a fundamental concurrency problem that can be exploited by local attackers to achieve privilege escalation or system disruption. The vulnerability manifests when manipulating the HDLC (High-Level Data Link Control) line discipline, which is used for serial communication protocols in Unix-like systems. The race condition occurs during the handling of line discipline changes, where multiple threads or processes can simultaneously access and modify the same data structures without proper synchronization mechanisms.
The technical flaw stems from insufficient locking mechanisms during the HDLC line discipline configuration process. When a local user attempts to set the HDLC line discipline on a tty device, the kernel's implementation fails to properly serialize access to critical data structures that manage the line discipline state. This lack of proper synchronization creates a window where concurrent operations can lead to memory corruption. The specific vulnerability results in a double free condition, where the same memory block gets deallocated twice, leading to undefined behavior that can be exploited to execute arbitrary code with elevated privileges. This type of vulnerability falls under CWE-362, which specifically addresses race conditions in software design where multiple threads or processes access shared resources without proper mutual exclusion.
The operational impact of this vulnerability extends beyond simple privilege escalation to encompass potential system stability issues that could result in denial of service conditions. Local attackers with minimal system access can exploit this weakness to either gain root privileges or crash the system entirely through memory corruption. The double free condition creates a scenario where the kernel's memory allocator becomes corrupted, potentially allowing attackers to manipulate heap metadata or overwrite critical kernel structures. This vulnerability is particularly dangerous in multi-user environments where attackers might already have access to user accounts but lack administrative privileges. The exploitation requires local access and knowledge of the specific tty device manipulation techniques, but once achieved, the impact can be severe enough to compromise entire system operations.
Mitigation strategies for CVE-2017-2636 primarily focus on applying the official kernel patches released by the Linux kernel development team, which address the underlying race condition through proper locking mechanisms and synchronization. System administrators should prioritize updating to kernel versions 4.10.2 or later where the vulnerability has been resolved. Additionally, implementing proper access controls and limiting local user privileges can reduce the attack surface, though this does not eliminate the vulnerability itself. Security monitoring should include detection of unusual tty device manipulations and line discipline changes that might indicate exploitation attempts. The remediation process should also involve comprehensive system hardening measures, including disabling unnecessary serial communication features and implementing strict privilege separation policies. Organizations should consider deploying kernel integrity monitoring solutions to detect and alert on unauthorized kernel modifications that might attempt to bypass the vulnerability fixes. This vulnerability demonstrates the critical importance of proper concurrency control in kernel space operations and aligns with ATT&CK technique T1068, which covers local privilege escalation through kernel vulnerabilities.