CVE-2020-11935 in Linux
Summary
by MITRE • 04/07/2023
It was discovered that aufs improperly managed inode reference counts in the vfsub_dentry_open() method. A local attacker could use this vulnerability to cause a denial of service attack.
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Analysis
by VulDB Data Team • 04/24/2023
The vulnerability identified as CVE-2020-11935 resides within the advanced union filesystem implementation known as aufs which is commonly used in Linux kernel environments to merge multiple filesystems into a single virtual filesystem. This flaw specifically manifests in the vfsub_dentry_open() method where the kernel fails to properly manage inode reference counts during file operations. The improper handling of these reference counts creates a scenario where the kernel's memory management becomes inconsistent, leading to potential resource exhaustion and system instability.
This vulnerability represents a classic case of improper resource management that falls under the CWE-404 category of Uncontrolled Resource Consumption. The flaw occurs when the aufs filesystem attempts to open a dentry object through the vfsub_dentry_open() function without maintaining proper reference counting mechanisms for the underlying inodes. When multiple file operations occur in rapid succession or in specific patterns, the reference count values become corrupted or improperly maintained, causing the kernel to lose track of which inodes are actively in use versus those that should be freed.
The operational impact of this vulnerability is significant for local attackers who can exploit it to perform denial of service attacks against systems running affected kernel versions. The attack vector requires local system access, making it a privilege escalation concern rather than a remote vulnerability. However, the potential for system instability remains high as the corrupted reference counts can lead to memory leaks, kernel panics, or complete system lockups. The vulnerability affects systems that utilize aufs filesystems, which are common in containerized environments, embedded systems, and various Linux distributions that support union filesystems for layered file access.
From a cybersecurity perspective, this vulnerability aligns with ATT&CK technique T1499.004 which involves network denial of service attacks. While the attack is local rather than network-based, the principle of causing system unavailability remains consistent. The vulnerability demonstrates how seemingly minor flaws in kernel-level filesystem operations can have cascading effects on system stability and availability. The exploitation pattern typically involves repeated file opening and closing operations that trigger the reference counting inconsistency, potentially leading to the exhaustion of available inodes or memory resources. Organizations using aufs filesystems should prioritize patching this vulnerability as it represents a straightforward path to system disruption that could be exploited in environments where local access is possible.
Mitigation strategies should focus on updating kernel versions to those containing the patched aufs implementation, which properly handles inode reference counting in the affected method. System administrators should also implement monitoring solutions to detect unusual filesystem behavior patterns that might indicate exploitation attempts. Additionally, reducing the attack surface by limiting local user access and implementing proper access controls can help prevent unauthorized exploitation of this vulnerability. The fix typically involves ensuring that reference counts are properly incremented and decremented during file operations, preventing the accumulation of stale references that could lead to resource exhaustion or system instability.