CVE-2021-4083 in Linux
Summary
by MITRE • 01/18/2022
A read-after-free memory flaw was found in the Linux kernel's garbage collection for Unix domain socket file handlers in the way users call close() and fget() simultaneously and can potentially trigger a race condition. This flaw allows a local user to crash the system or escalate their privileges on the system. This flaw affects Linux kernel versions prior to 5.16-rc4.
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Analysis
by VulDB Data Team • 10/14/2024
The vulnerability identified as CVE-2021-4083 represents a critical read-after-free memory flaw within the Linux kernel's garbage collection mechanism for Unix domain socket file handlers. This issue manifests in a specific race condition scenario where simultaneous invocation of close() and fget() system calls creates a temporal window for memory access violations. The flaw exists in the kernel's handling of file descriptor management during socket cleanup operations, specifically affecting the interaction between reference counting and memory deallocation processes. The vulnerability operates at the kernel level, making it particularly dangerous as it can be exploited by local users with minimal privileges to achieve system compromise.
The technical implementation of this vulnerability stems from improper synchronization mechanisms within the kernel's Unix domain socket implementation. When a process attempts to close a socket file descriptor while another process simultaneously calls fget() on the same file descriptor, the kernel's garbage collection logic fails to properly handle the concurrent access pattern. This race condition results in a situation where memory that has been freed by the close() operation can still be accessed by the fget() operation, leading to a read-after-free condition. The flaw is categorized under CWE-416 as a use-after-free vulnerability, though it specifically manifests as a read-after-free due to the nature of how the kernel handles the memory lifecycle during concurrent operations. The vulnerability is particularly insidious because it can be triggered through legitimate system calls that are commonly used by applications and system processes.
The operational impact of CVE-2021-4083 extends beyond simple system crashes to include potential privilege escalation capabilities that make it a severe security concern for Linux systems. Local attackers can exploit this vulnerability to cause system instability through kernel panics and crashes, but more critically, they can leverage the memory corruption to elevate their privileges to root level access. This privilege escalation occurs because the memory corruption can be manipulated to redirect execution flow or modify kernel data structures that control access controls. The vulnerability affects all Linux kernel versions prior to 5.16-rc4, which represents a significant portion of deployed systems, making the impact widespread across various server and desktop environments. The flaw's exploitation requires only local user access and does not require network connectivity, making it particularly dangerous in multi-user environments or systems where untrusted users have access to local accounts.
Mitigation strategies for CVE-2021-4083 primarily focus on kernel version updates and system hardening measures. The most effective immediate solution is upgrading to Linux kernel version 5.16-rc4 or later, where the race condition has been properly addressed through enhanced synchronization mechanisms and improved memory management for socket file descriptors. System administrators should prioritize patching affected systems as this vulnerability can be exploited without user interaction and has a high potential for system compromise. Additional mitigations include implementing proper access controls to limit local user privileges, monitoring for unusual system behavior that might indicate exploitation attempts, and applying kernel lockdown mechanisms where appropriate. The vulnerability aligns with ATT&CK technique T1068 which covers 'Local Privilege Escalation' and T1499 which covers 'Endpoint Denial of Service', demonstrating both the privilege escalation and system stability impact potential of this flaw. Organizations should also consider implementing kernel module hardening and runtime monitoring to detect potential exploitation attempts before they can cause significant damage to system integrity or availability.