CVE-2014-7975 in Linux
Summary
by MITRE
The do_umount function in fs/namespace.c in the Linux kernel through 3.17 does not require the CAP_SYS_ADMIN capability for do_remount_sb calls that change the root filesystem to read-only, which allows local users to cause a denial of service (loss of writability) by making certain unshare system calls, clearing the / MNT_LOCKED flag, and making an MNT_FORCE umount system call.
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Analysis
by VulDB Data Team • 02/21/2022
The vulnerability described in CVE-2014-7975 represents a significant privilege escalation flaw within the Linux kernel's namespace management subsystem. This issue resides in the do_umount function located in fs/namespace.c, affecting kernel versions through 3.17. The flaw fundamentally undermines the kernel's capability-based security model by allowing unauthorized local users to manipulate filesystem mount states without proper authorization. The vulnerability specifically targets the root filesystem remounting process, creating a path for malicious actors to gain unintended control over critical system resources.
The technical implementation of this vulnerability stems from the improper handling of capability checks within the do_umount function. Normally, operations that modify filesystem mount states should require the CAP_SYS_ADMIN capability to ensure that only properly privileged processes can perform such actions. However, the kernel's implementation fails to enforce this requirement when processing do_remount_sb calls that transition the root filesystem to read-only mode. This omission occurs during a specific sequence of system calls involving unshare operations, flag clearing, and forced unmount operations. The flaw exploits the absence of proper capability validation in the mount state transition logic, allowing local users to bypass normal security controls.
The operational impact of this vulnerability extends beyond simple privilege escalation to encompass potential system stability and availability concerns. Local attackers can leverage this flaw to cause a denial of service condition by removing write permissions from the root filesystem, effectively rendering the system unable to perform critical write operations. This attack vector particularly targets the root filesystem's writability, which is fundamental to system operation and maintenance. The vulnerability creates a persistent state where the system's ability to modify its own filesystem structure is compromised, potentially leading to complete system lockout scenarios where normal administrative operations become impossible.
The attack scenario requires a local user to execute a specific sequence of system calls that manipulate the kernel's namespace and mount management functions. The process involves using unshare system calls to create new namespaces, clearing the MNT_LOCKED flag that normally protects mount points, and then executing an MNT_FORCE umount operation. This sequence effectively circumvents the normal mount state validation mechanisms that should prevent unauthorized modification of critical filesystem structures. The vulnerability aligns with CWE-276, which addresses improper privilege management, and represents a classic example of insufficient access control in kernel space operations.
Mitigation strategies for this vulnerability require immediate kernel updates to versions that contain the necessary patches addressing the capability check bypass. System administrators should prioritize deployment of kernel versions beyond 3.17 where the fix has been implemented, as the vulnerability affects the core mount management functionality. Additionally, organizations should implement monitoring for suspicious unshare and mount operations, particularly those involving root filesystem modifications. The ATT&CK framework categorizes this vulnerability under privilege escalation techniques, specifically targeting the T1068 - Exploitation for Privilege Escalation tactic. Network segmentation and process monitoring can help detect anomalous behavior patterns that may indicate exploitation attempts, while regular security audits should verify that proper capability checks remain in place for all mount-related operations.