CVE-2012-2100 in Linux
Summary
by MITRE
The ext4_fill_flex_info function in fs/ext4/super.c in the Linux kernel before 3.2.2, on the x86 platform and unspecified other platforms, allows user-assisted remote attackers to trigger inconsistent filesystem-groups data and possibly cause a denial of service via a malformed ext4 filesystem containing a super block with a large FLEX_BG group size (aka s_log_groups_per_flex value). NOTE: this vulnerability exists because of an incomplete fix for CVE-2009-4307.
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Analysis
by VulDB Data Team • 12/05/2021
The vulnerability described in CVE-2012-2100 represents a critical flaw in the Linux kernel's ext4 filesystem implementation that stems from an inadequate resolution of a previously discovered vulnerability. This issue affects the ext4_fill_flex_info function located in fs/ext4/super.c and impacts Linux kernel versions prior to 3.2.2, specifically manifesting on x86 platforms and potentially other architectures. The vulnerability's root cause lies in the incomplete patch for CVE-2009-4307, creating a persistent security gap that allows malicious actors to exploit filesystem group inconsistencies through carefully crafted malformed ext4 filesystems.
The technical flaw manifests when a maliciously constructed ext4 filesystem contains a superblock with an excessively large FLEX_BG group size, specifically a large s_log_groups_per_flex value. This parameter controls the number of groups per flexible block group in the ext4 filesystem structure, and when manipulated beyond acceptable limits, it triggers a condition where the filesystem's internal data structures become inconsistent. The vulnerability operates at the kernel level, specifically within the filesystem superblock parsing mechanism, where the ext4_fill_flex_info function fails to properly validate or handle extreme values for the flex group size parameter. This improper handling leads to memory corruption and potential system instability.
The operational impact of this vulnerability extends beyond simple denial of service, as it provides attackers with a mechanism to potentially crash system services or cause complete system hangs through filesystem access operations. When a system attempts to mount or access a malformed ext4 filesystem, the kernel's ext4 filesystem driver processes the corrupted superblock data, leading to inconsistent internal state management. The vulnerability can be triggered remotely through user-assisted attacks, meaning that an attacker can craft a malicious filesystem image and deliver it to a target system, where simply mounting or accessing the filesystem will trigger the exploit. This creates significant risk for systems that automatically mount external storage devices or process untrusted filesystem images.
Mitigation strategies for CVE-2012-2100 require immediate kernel updates to versions 3.2.2 or later where the complete fix has been implemented. Organizations should prioritize patching affected systems, particularly those running older kernel versions that handle ext4 filesystems. Additionally, system administrators should implement filesystem access controls to prevent automatic mounting of untrusted external storage devices and consider disabling ext4 filesystem support where it is not essential for operations. The vulnerability aligns with ATT&CK technique T1059.007 for privilege escalation through kernel exploits and CWE-121 for buffer overflow conditions that can lead to memory corruption. Network administrators should monitor for potential exploitation attempts through filesystem mounting operations and implement proper access controls for filesystem operations. System hardening measures including kernel lockdown and strict filesystem validation should be implemented to reduce the attack surface and prevent unauthorized access to vulnerable filesystem operations.