CVE-2018-20856 in Linux
Summary
by MITRE
An issue was discovered in the Linux kernel before 4.18.7. In block/blk-core.c, there is an __blk_drain_queue() use-after-free because a certain error case is mishandled.
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Analysis
by VulDB Data Team • 11/13/2023
The vulnerability identified as CVE-2018-20856 represents a critical use-after-free condition within the Linux kernel's block layer subsystem. This flaw exists in the block/blk-core.c file and affects kernel versions prior to 4.18.7, creating a significant security risk for systems relying on kernel-based storage operations. The issue stems from improper error handling within the __blk_drain_queue() function, which governs the draining of I/O queues in the block layer. When specific error conditions occur during queue processing, the function fails to properly manage memory references, leading to scenarios where freed memory locations are accessed after deallocation.
The technical implementation of this vulnerability involves the kernel's block subsystem managing I/O requests through queue operations that require careful memory management. During normal operation, the __blk_drain_queue() function processes pending requests and ensures proper cleanup of resources. However, when error conditions arise during this process, the function does not correctly handle the reference counting or memory deallocation sequence, resulting in a situation where a memory block that has been freed continues to be referenced by subsequent operations. This use-after-free condition creates a potential exploitation vector for privilege escalation attacks, as malicious actors could manipulate the system into executing arbitrary code through controlled memory corruption.
The operational impact of CVE-2018-20856 extends beyond simple system instability, presenting serious security implications for kernel-based systems. Attackers exploiting this vulnerability could potentially gain elevated privileges within the kernel space, allowing them to execute arbitrary code with the highest system privileges. The vulnerability affects systems running affected kernel versions and poses risks to enterprise environments, cloud infrastructure, and embedded systems that rely on Linux kernel storage operations. The flaw's impact is particularly concerning because it operates within the core block I/O subsystem, which is fundamental to virtually all storage operations and system functionality. This makes the vulnerability particularly dangerous as it could be exploited through various attack vectors that trigger I/O queue operations, including file system operations, disk access, and storage management functions.
Mitigation strategies for CVE-2018-20856 focus primarily on kernel version updates and system hardening measures. The most effective remediation involves upgrading to Linux kernel version 4.18.7 or later, where the memory management error handling has been corrected. System administrators should prioritize patching affected systems and implementing automated update mechanisms to prevent exploitation. Additionally, monitoring systems should be configured to detect unusual I/O queue operations that might indicate exploitation attempts. The vulnerability aligns with CWE-416, which categorizes use-after-free conditions as a common memory safety issue, and could potentially map to ATT&CK technique T1068, which involves exploiting legitimate credentials and privileges. Organizations should also implement kernel hardening measures such as stack canaries, address space layout randomization, and kernel module signing to reduce the effectiveness of potential exploitation attempts. Regular security audits and vulnerability assessments should include checks for this specific kernel vulnerability to ensure comprehensive system protection against persistent threats.