CVE-2011-2695 in Linux
Summary
by MITRE
Multiple off-by-one errors in the ext4 subsystem in the Linux kernel before 3.0-rc5 allow local users to cause a denial of service (BUG_ON and system crash) by accessing a sparse file in extent format with a write operation involving a block number corresponding to the largest possible 32-bit unsigned integer.
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Analysis
by VulDB Data Team • 11/15/2021
The vulnerability described in CVE-2011-2695 represents a critical flaw in the Linux kernel's ext4 filesystem implementation that manifests through multiple off-by-one errors within the subsystem responsible for handling sparse files in extent format. This particular weakness exists in kernel versions prior to 3.0-rc5 and specifically targets the way the ext4 filesystem processes write operations on sparse files. The vulnerability occurs when a local user attempts to perform a write operation on a sparse file that utilizes the extent format, where the operation involves a block number that corresponds to the maximum possible value of a 32-bit unsigned integer, which is 4294967295. This specific block number triggers a condition where the kernel's internal validation logic fails to properly account for the boundary conditions, leading to a cascading failure in the system's memory management and file operations.
The technical nature of this vulnerability stems from improper boundary checking within the ext4 filesystem code that handles extent-based file storage. When a write operation is initiated on a sparse file using extent format, the kernel's extent allocation and management routines fail to correctly validate the maximum block number that can be processed. This off-by-one error creates a condition where the kernel's BUG_ON macro is triggered, which is a debugging mechanism designed to catch internal inconsistencies in kernel code. The BUG_ON condition causes the kernel to immediately terminate the operation and initiate a system crash, effectively rendering the system unusable. This type of vulnerability falls under the CWE-129 category of Improper Validation of Array Index, which specifically addresses situations where array indices or similar counters are not properly validated against their legitimate bounds. The flaw represents a classic example of how seemingly minor boundary condition errors can result in catastrophic system failures.
The operational impact of this vulnerability extends beyond simple denial of service, as it provides a mechanism for local attackers to destabilize systems running vulnerable kernel versions. Since the vulnerability requires only local user access, it can be exploited by any user with access to the system, making it particularly dangerous in multi-user environments where privilege escalation might not be necessary. The system crash resulting from this vulnerability can cause complete system instability, requiring manual intervention to restore normal operations. This type of attack directly violates the principle of least privilege and can be leveraged as part of broader exploitation campaigns, particularly when combined with other vulnerabilities in the kernel's filesystem subsystem. The attack vector is relatively simple to execute, requiring only a specific write operation on a sparse file with a particular block number, making it an attractive target for attackers seeking to disrupt system availability. The vulnerability also aligns with ATT&CK technique T1499.004 which focuses on Network Denial of Service, though in this case the attack targets local system resources rather than network infrastructure.
Mitigation strategies for CVE-2011-2695 primarily involve upgrading to kernel versions 3.0-rc5 or later, where the off-by-one errors have been corrected through proper boundary validation and array index checking. System administrators should prioritize patching vulnerable systems, particularly those running in production environments where availability is critical. Additional protective measures include implementing strict file access controls and monitoring for unusual file operations on sparse files, though these measures are secondary to the primary requirement of kernel patching. The vulnerability demonstrates the importance of thorough testing of boundary conditions in kernel code and highlights the need for comprehensive validation of all input parameters in system-level operations. Organizations should also consider implementing automated patch management systems to ensure timely deployment of security updates. The fix implemented in kernel 3.0-rc5 specifically addresses the array index validation issues by ensuring that the maximum 32-bit unsigned integer value is properly handled within the extent allocation routines, preventing the triggering of the BUG_ON condition that caused the system crashes.