CVE-2012-5374 in Linux
Summary
by MITRE
The CRC32C feature in the Btrfs implementation in the Linux kernel before 3.8-rc1 allows local users to cause a denial of service (extended runtime of kernel code) by creating many different files whose names are associated with the same CRC32C hash value.
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Analysis
by VulDB Data Team • 04/20/2021
The vulnerability described in CVE-2012-5374 represents a significant denial of service weakness within the Btrfs filesystem implementation of the Linux kernel. This flaw specifically targets the CRC32C hash calculation mechanism that Btrfs employs for file name management and internal data structures. The issue arises from the inherent properties of the CRC32C algorithm when used in a hash table context, where hash collisions can occur and potentially be exploited by malicious actors. This vulnerability falls under the category of computational complexity attacks, where an attacker can manipulate system resources to create extended processing times for kernel operations. The Btrfs filesystem, designed as a next-generation storage solution for Linux, incorporates advanced features including copy-on-write semantics, snapshots, and integrated data integrity checking through CRC32C calculations.
The technical flaw stems from the implementation of hash table collision handling within the Btrfs kernel module's file name processing routines. When multiple file names hash to the same CRC32C value, the system must resolve these collisions through additional computational overhead, typically through linked list traversal or other collision resolution mechanisms. In the vulnerable versions of the Linux kernel before 3.8-rc1, the hash table implementation did not properly limit the number of entries that could be associated with a single hash value, allowing an attacker to deliberately create numerous files with carefully crafted names that would all map to the same hash bucket. This results in a degradation of performance where operations that should normally complete in constant time instead require linear time proportional to the number of collisions. The vulnerability specifically affects the kernel's runtime performance rather than causing system crashes or memory corruption, making it a subtle but impactful denial of service vector.
The operational impact of this vulnerability extends beyond simple performance degradation to potentially render systems unusable under sustained attack conditions. Local users with access to the filesystem can exploit this weakness by creating thousands of files with names designed to collide in the CRC32C hash space, causing the kernel to spend excessive CPU cycles resolving hash collisions during file operations such as directory listings, file creation, or metadata queries. This extended runtime behavior affects system responsiveness and can potentially impact other kernel subsystems that depend on timely completion of filesystem operations. The attack vector is particularly concerning because it requires no special privileges beyond local access to the filesystem and can be executed with minimal resources, making it an attractive method for system administrators to identify potential DoS threats. The vulnerability demonstrates the importance of proper hash table design and collision handling in kernel space implementations, where performance characteristics directly impact system stability and availability.
Mitigation strategies for this vulnerability involve upgrading to Linux kernel versions 3.8-rc1 or later, where the Btrfs implementation includes improved hash table collision handling and better resource management. System administrators should also implement monitoring for unusual filesystem activity patterns that might indicate exploitation attempts, particularly around directory creation and file naming operations. The fix typically involves implementing proper hash table sizing, collision resolution algorithms, and resource limits to prevent any single hash bucket from accumulating excessive entries. This vulnerability aligns with CWE-778, which covers insufficient logging of resource consumption, and relates to ATT&CK technique T1499.004, which describes resource exhaustion attacks. Organizations should also consider implementing filesystem access controls and monitoring to detect and prevent unauthorized file creation patterns that could be used to exploit this vulnerability, ensuring that the system maintains adequate performance even under malicious conditions.