CVE-2016-4581 in Linux
Summary
by MITRE
fs/pnode.c in the Linux kernel before 4.5.4 does not properly traverse a mount propagation tree in a certain case involving a slave mount, which allows local users to cause a denial of service (NULL pointer dereference and OOPS) via a crafted series of mount system calls.
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Analysis
by VulDB Data Team • 08/19/2022
The vulnerability described in CVE-2016-4581 resides within the Linux kernel's file system subsystem, specifically in the fs/pnode.c file which handles mount propagation operations. This flaw represents a critical issue in kernel space that can be exploited by local attackers to disrupt system operations through carefully crafted mount commands. The vulnerability affects Linux kernel versions prior to 4.5.4, making it a significant concern for systems running older kernel releases.
The technical root cause of this vulnerability lies in the improper traversal of the mount propagation tree structure when handling slave mount operations. In Linux, mount propagation allows filesystem mounts to be shared across different mount points, creating a hierarchical relationship between mounts. When a mount is configured as a slave, it should not receive propagation events from its master, but the kernel's implementation contains a flaw in the traversal logic that fails to properly handle edge cases. This results in a situation where a NULL pointer reference occurs during the mount operation processing, leading to an immediate system crash.
The operational impact of this vulnerability manifests as a denial of service condition that can be triggered by any local user with the ability to execute mount system calls. When the crafted sequence of mount operations is executed, the kernel encounters a NULL pointer dereference in the pnode.c file, which generates an OOPS message and ultimately causes the system to crash or reboot. This type of vulnerability is particularly dangerous in multi-user environments or when the system is running critical services, as it can be exploited to disrupt system availability without requiring special privileges beyond standard user access.
This vulnerability maps to CWE-476 which describes NULL pointer dereference, and aligns with several ATT&CK techniques including privilege escalation through system resource exhaustion and denial of service attacks. The attack surface is primarily through the mount system call interface which is commonly available to local users, making this a prevalent threat in environments where users have shell access. The exploitation requires no special privileges or network access, as it operates entirely within the local system context and leverages existing kernel functionality.
The recommended mitigation strategy involves upgrading to Linux kernel version 4.5.4 or later, where the mount propagation tree traversal logic has been corrected. System administrators should prioritize this update, particularly in production environments where system availability is critical. Additionally, monitoring for unusual mount operations or system crashes can help detect exploitation attempts. Organizations should also implement proper access controls to limit local user privileges where possible, reducing the attack surface for this particular vulnerability. The fix addresses the core traversal logic issue in the kernel's mount propagation handling, ensuring that slave mount relationships are properly maintained without causing NULL pointer dereferences during normal operation.