CVE-2020-29660 in Linux
Summary
by MITRE • 12/09/2020
A locking inconsistency issue was discovered in the tty subsystem of the Linux kernel through 5.9.13. drivers/tty/tty_io.c and drivers/tty/tty_jobctrl.c may allow a read-after-free attack against TIOCGSID, aka CID-c8bcd9c5be24.
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Analysis
by VulDB Data Team • 12/15/2020
The vulnerability identified as CVE-2020-29660 represents a critical locking inconsistency within the Linux kernel's terminal subsystem, specifically affecting versions through 5.9.13. This flaw resides in the tty (teletypewriter) subsystem which manages terminal device interfaces and process group control. The issue manifests in two primary files: drivers/tty/tty_io.c and drivers/tty/tty_jobctrl.c, where improper synchronization mechanisms create conditions that can be exploited by malicious actors. The vulnerability is particularly concerning because it enables a read-after-free attack pattern against the TIOCGSID ioctl command, which is used to retrieve the session ID of a terminal session.
The technical root cause of this vulnerability stems from inadequate locking mechanisms during the processing of TIOCGSID requests within the tty subsystem. When a process attempts to query the session ID of a terminal device, the kernel's tty subsystem must ensure proper synchronization between different code paths that handle terminal operations and process group management. The locking inconsistency allows for a race condition scenario where a terminal device structure might be freed from memory while another thread or process attempts to read from it, creating a classic read-after-free vulnerability. This type of flaw falls under CWE-367, which specifically addresses Time-of-Check to Time-of-Use (TOCTOU) vulnerabilities and improper locking scenarios that can lead to memory corruption.
The operational impact of CVE-2020-29660 extends beyond simple memory corruption, as it can potentially enable privilege escalation attacks within the Linux kernel environment. An attacker who successfully exploits this vulnerability could gain unauthorized access to terminal sessions, manipulate process group memberships, or potentially execute arbitrary code with kernel-level privileges. The vulnerability is particularly dangerous because it affects core kernel functionality that is essential for system operations, making it a prime target for exploitation in both local and remote attack scenarios. From an ATT&CK framework perspective, this vulnerability maps to T1068 (Local Privilege Escalation) and T1059 (Command and Scripting Interpreter) as attackers could leverage the kernel-level access to execute malicious commands and maintain persistence.
Mitigation strategies for CVE-2020-29660 require immediate kernel updates to versions that have patched the locking inconsistency in the tty subsystem. System administrators should prioritize applying security patches from their respective kernel vendors, as this vulnerability affects the fundamental terminal management capabilities of Linux systems. Additionally, monitoring for unusual terminal activity and process group changes can help detect potential exploitation attempts. Organizations should also implement proper access controls and privilege separation to limit the potential impact of successful exploitation. The fix typically involves implementing proper mutex or spinlock mechanisms around the critical sections of code that handle TIOCGSID operations, ensuring that terminal device structures remain valid throughout the entire operation. This vulnerability demonstrates the critical importance of proper synchronization in kernel-level code and highlights the need for thorough security auditing of core subsystems that handle inter-process communication and resource management.