CVE-2014-9922 in Linux
Summary
by MITRE
The eCryptfs subsystem in the Linux kernel before 3.18 allows local users to gain privileges via a large filesystem stack that includes an overlayfs layer, related to fs/ecryptfs/main.c and fs/overlayfs/super.c.
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Analysis
by VulDB Data Team • 11/27/2022
The eCryptfs subsystem in the Linux kernel prior to version 3.18 contained a critical privilege escalation vulnerability that exploited the interaction between eCryptfs and overlayfs filesystem layers. This vulnerability arose from insufficient validation of filesystem stack depths during the mounting process, creating a condition where local attackers could manipulate the kernel's handling of nested filesystem operations. The flaw specifically affected the kernel's filesystem layer management, particularly in how it processed stacked filesystems that included overlayfs as an intermediate layer. The vulnerability was rooted in the kernel's failure to properly enforce limits on the depth of filesystem stacking operations, which allowed malicious users to construct arbitrarily deep filesystem stacks that could trigger unexpected behavior in the kernel's memory management and privilege handling mechanisms.
The technical implementation of this vulnerability involved exploiting the interaction between fs/ecryptfs/main.c and fs/overlayfs/super.c kernel source files, where the eCryptfs subsystem failed to validate the maximum depth of filesystem stacks when overlayfs was involved. When a user created a complex filesystem stack with multiple layers including overlayfs, the kernel's handling of these nested operations could lead to memory corruption or privilege escalation conditions. The vulnerability was particularly dangerous because it leveraged legitimate kernel functionality while exploiting a design oversight in the filesystem stacking validation logic. Attackers could construct a specific sequence of mount operations that would cause the kernel to exceed normal stack depth limits, potentially leading to kernel memory corruption or privilege escalation to root level access.
The operational impact of this vulnerability was significant for Linux systems running kernel versions before 3.18, as it provided local attackers with a straightforward method to escalate privileges without requiring special permissions or complex exploitation techniques. The vulnerability could be exploited by any user with access to the system, making it particularly concerning for multi-user environments or systems where untrusted users had shell access. The attack vector was relatively simple to implement, requiring only the ability to create and mount filesystems, which made it accessible to a wide range of potential attackers. Systems using eCryptfs for encrypted filesystems were particularly at risk, as the vulnerability could be triggered through normal filesystem operations rather than requiring special conditions or complex attack chains.
Mitigation strategies for this vulnerability required immediate kernel updates to version 3.18 or later, where the kernel developers implemented proper validation of filesystem stack depths and strengthened the checks in both eCryptfs and overlayfs subsystems. Organizations should have also implemented monitoring for unusual filesystem mounting patterns and ensured that filesystem stacking operations were properly restricted in security policies. The fix addressed the root cause by introducing proper depth limiting mechanisms in the kernel's filesystem layer management, preventing the construction of excessively deep filesystem stacks that could trigger the privilege escalation conditions. System administrators should have also reviewed and restricted the use of overlayfs and eCryptfs in environments where untrusted users had access, implementing additional security controls to prevent exploitation attempts.
This vulnerability aligns with CWE-121, which describes stack-based buffer overflow conditions, and represents a classic example of improper input validation in kernel space operations. The ATT&CK framework categorizes this as privilege escalation through kernel vulnerabilities, specifically targeting the 'Exploitation for Privilege Escalation' technique. The vulnerability demonstrates how seemingly legitimate kernel functionality can be exploited when proper validation and depth limiting mechanisms are absent, highlighting the importance of defensive programming practices in kernel space. The issue also relates to the broader category of filesystem manipulation attacks that target kernel subsystems, making it relevant to various security frameworks that emphasize kernel-level attack surface reduction and proper resource management in operating system kernels.