CVE-2020-29534 in Linux
Summary
by MITRE • 12/04/2020
An issue was discovered in the Linux kernel before 5.9.3. io_uring takes a non-refcounted reference to the files_struct of the process that submitted a request, causing execve() to incorrectly optimize unshare_fd(), aka CID-0f2122045b94.
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Analysis
by VulDB Data Team • 12/12/2020
The vulnerability identified as CVE-2020-29534 represents a critical flaw in the Linux kernel's io_uring subsystem that affects versions prior to 5.9.3. This issue stems from improper handling of file descriptor references within the io_uring framework, specifically involving the files_struct data structure that manages file descriptors for processes. The flaw manifests when io_uring operations maintain a non-refcounted reference to a process's files_struct, creating a scenario where the kernel's execve() system call incorrectly optimizes the unshare_fd() operation. This misoptimization creates a potential race condition and memory management inconsistency that could be exploited by malicious actors to gain unauthorized access or cause system instability.
The technical root cause of this vulnerability lies in the improper reference counting mechanism within the io_uring implementation. When a process submits an io_uring request, the subsystem maintains a reference to the process's files_struct without properly incrementing the reference count. This creates a situation where the files_struct could be freed or modified while io_uring operations are still referencing it, particularly during execve() operations that attempt to optimize file descriptor handling. The unshare_fd() optimization, which is designed to improve performance by avoiding unnecessary file descriptor duplication, becomes compromised when the files_struct reference is not properly managed. This mismanagement leads to a potential use-after-free condition or inconsistent state that could be leveraged for privilege escalation or denial of service attacks.
The operational impact of CVE-2020-29534 extends beyond simple system instability, potentially enabling attackers to exploit the improper reference handling for privilege escalation within the Linux kernel. Since io_uring is designed for high-performance asynchronous I/O operations, this vulnerability affects systems that rely heavily on asynchronous operations, particularly those running applications that utilize io_uring for network or storage I/O. The vulnerability is especially concerning in server environments where execve() operations are frequent and where applications might be using io_uring for performance-critical operations. Attackers could potentially trigger this condition through carefully crafted io_uring requests that would cause the kernel to incorrectly handle file descriptor sharing during process execution, leading to either unauthorized privilege elevation or system crash conditions that could be exploited for persistent access.
This vulnerability aligns with CWE-415, which addresses double free errors and improper resource management, and exhibits characteristics similar to CWE-416, dealing with use after free conditions. The flaw also relates to ATT&CK technique T1068, which covers local privilege escalation through kernel vulnerabilities, and T1547, focusing on privilege escalation via kernel exploits. The improper reference counting mechanism creates a pathway for attackers to manipulate kernel memory structures, potentially leading to full system compromise. Organizations running Linux systems with kernel versions prior to 5.9.3 should consider this vulnerability as a high-priority threat requiring immediate mitigation, particularly in environments where io_uring is actively used or where the system may be subject to untrusted input processing. The fix implemented in kernel version 5.9.3 addresses the reference counting issue by ensuring proper management of files_struct references during io_uring operations, preventing the incorrect optimization that led to the vulnerability.
The remediation strategy for CVE-2020-29534 centers on upgrading to Linux kernel version 5.9.3 or later, which contains the necessary patches to properly manage the files_struct references within io_uring operations. System administrators should prioritize this upgrade across all affected systems, particularly those running io_uring-enabled applications or services that handle untrusted input. Additionally, organizations should implement monitoring for unusual execve() operations or io_uring activity that might indicate exploitation attempts. The vulnerability demonstrates the critical importance of proper reference counting in kernel space operations and highlights the need for thorough testing of kernel subsystems that handle resource management and process operations. Security teams should also consider implementing kernel hardening measures and runtime protections to detect and prevent exploitation attempts targeting this specific vulnerability pattern.