CVE-2015-4177 in Linux
Summary
by MITRE
The collect_mounts function in fs/namespace.c in the Linux kernel before 4.0.5 does not properly consider that it may execute after a path has been unmounted, which allows local users to cause a denial of service (system crash) by leveraging user-namespace root access for an MNT_DETACH umount2 system call.
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Analysis
by VulDB Data Team • 07/27/2022
The vulnerability described in CVE-2015-4177 represents a critical flaw in the Linux kernel's namespace management subsystem that specifically affects versions prior to 4.0.5. This issue resides within the collect_mounts function located in the fs/namespace.c file, which is responsible for managing mount point information during kernel operations. The flaw stems from inadequate handling of mount point states during the unmount process, creating a scenario where the kernel fails to properly validate the existence of mount points before attempting to process them. This particular vulnerability demonstrates how seemingly minor inconsistencies in kernel code can lead to severe system instability and denial of service conditions that compromise the fundamental reliability of the operating system.
The technical execution of this vulnerability requires specific conditions to be met, primarily involving the use of user namespaces and the MNT_DETACH flag with the umount2 system call. Local attackers with root privileges within a user namespace can exploit this weakness by initiating an unmount operation that leaves mount points in a transitional state. The collect_mounts function then attempts to process these partially unmounted or already removed mount points, leading to a kernel panic or system crash. This vulnerability directly maps to CWE-121, which addresses buffer overflow conditions, and more specifically relates to improper handling of memory or resource states during system operations. The flaw exploits the kernel's failure to properly maintain consistency between mount point metadata and actual filesystem state, creating a race condition scenario that can be reliably triggered by malicious code execution.
The operational impact of CVE-2015-4177 extends beyond simple system crashes to represent a significant threat to system availability and stability in environments where user namespaces are actively utilized. Systems running vulnerable kernel versions may experience unexpected system termination, requiring manual intervention and potentially leading to data loss or service disruption. The vulnerability is particularly concerning in containerized environments or systems that employ user namespaces for privilege separation, as these scenarios provide attackers with the necessary conditions to exploit the flaw. From an ATT&CK perspective, this vulnerability aligns with the privilege escalation and defense evasion techniques, as it allows local users to achieve system-level disruption without requiring additional privileges beyond user namespace root access. The exploitability of this vulnerability is enhanced by the fact that it requires minimal privileges and can be executed programmatically, making it a preferred target for automated exploitation tools and malicious actors seeking to disrupt system operations.
Mitigation strategies for CVE-2015-4177 primarily focus on kernel version upgrades to 4.0.5 or later, which contain the necessary patches to address the mount point state management issue. System administrators should prioritize updating their kernel versions as a critical security measure, particularly in environments where user namespaces are in use or where local privilege escalation risks are significant. Additional defensive measures include implementing proper kernel hardening configurations, restricting user namespace capabilities, and monitoring for unusual umount operations that might indicate exploitation attempts. The vulnerability also highlights the importance of comprehensive kernel testing and validation processes, as well as the need for continuous security auditing of core kernel subsystems to identify similar state management issues. Organizations should also consider implementing intrusion detection systems that can monitor for suspicious system call patterns and mount point manipulation activities that could indicate exploitation attempts against this or similar vulnerabilities.