CVE-2020-36557 in Linux
Summary
by MITRE • 07/21/2022
A race condition in the Linux kernel before 5.6.2 between the VT_DISALLOCATE ioctl and closing/opening of ttys could lead to a use-after-free.
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Analysis
by VulDB Data Team • 08/15/2022
This vulnerability exists within the Linux kernel's terminal subsystem and represents a critical race condition that could enable arbitrary code execution. The flaw occurs in the interaction between the VT_DISALLOCATE ioctl command and the manipulation of tty devices through open and close operations. When a process attempts to deallocate a virtual terminal while another process simultaneously opens or closes the corresponding tty device, the kernel fails to properly synchronize these operations, creating a window where memory structures can become invalid while still being referenced.
The technical implementation of this vulnerability stems from insufficient locking mechanisms within the kernel's virtual terminal driver code. When VT_DISALLOCATE is executed, it attempts to free memory associated with a tty structure while concurrent operations may still be accessing that same memory. This race condition specifically affects the tty driver's reference counting and memory management routines, where the kernel does not adequately prevent simultaneous access patterns that could lead to a use-after-free condition. The vulnerability is particularly dangerous because it allows an attacker to manipulate the timing of these operations to cause the kernel to dereference freed memory, potentially leading to privilege escalation or system crashes.
The operational impact of this vulnerability extends beyond simple system instability to include potential privilege escalation and denial of service scenarios. An attacker with access to a terminal session could exploit this race condition to execute arbitrary code with kernel privileges, effectively compromising the entire system. The vulnerability affects systems running Linux kernel versions prior to 5.6.2, making it particularly concerning for organizations maintaining older kernel versions or those with delayed patching schedules. The race condition is difficult to exploit reliably due to timing requirements, but when successful, it provides a pathway to kernel-level compromise that could be leveraged for persistent access or data exfiltration.
Mitigation strategies for this vulnerability primarily focus on immediate kernel updates to version 5.6.2 or later, which contain the necessary synchronization fixes. System administrators should prioritize patching critical systems and implementing automated patch management processes to prevent similar vulnerabilities from remaining unaddressed. Additional protective measures include monitoring for suspicious terminal activity patterns, implementing strict access controls for terminal devices, and employing kernel hardening techniques such as stack canaries and address space layout randomization. Organizations should also consider implementing intrusion detection systems that can identify anomalous tty access patterns that might indicate exploitation attempts. This vulnerability aligns with CWE-362, which describes race conditions in concurrent programming, and maps to ATT&CK technique T1059 for command and scripting interpreter usage that could be employed to exploit the privilege escalation path. The fix implemented in kernel 5.6.2 addresses the core synchronization issues by adding proper locking mechanisms around the tty deallocation and access operations, ensuring that memory structures remain valid during all concurrent operations.