CVE-2004-0427 in Linux
Summary
by MITRE
The do_fork function in Linux 2.4.x before 2.4.26, and 2.6.x before 2.6.6, does not properly decrement the mm_count counter when an error occurs after the mm_struct for a child process has been activated, which triggers a memory leak that allows local users to cause a denial of service (memory exhaustion) via the clone (CLONE_VM) system call.
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Analysis
by VulDB Data Team • 09/02/2019
The vulnerability described in CVE-2004-0427 represents a critical memory management flaw within the Linux kernel's process creation mechanism. This issue affects kernel versions 2.4.x prior to 2.4.26 and 2.6.x prior to 2.6.6, specifically targeting the do_fork function which handles the creation of new processes through the clone system call with the CLONE_VM flag. The vulnerability stems from improper handling of memory management counters during error conditions in the fork operation, creating a persistent memory leak that can be exploited by local attackers.
The technical flaw occurs within the kernel's memory management subsystem where the mm_count counter, which tracks the number of processes sharing a memory management structure, fails to be properly decremented when an error condition arises after the mm_struct for a child process has already been initialized. This counter is essential for proper memory accounting and cleanup, as it ensures that memory resources are released when no longer needed. When the error occurs during the fork process, the system incorrectly maintains the reference count, leading to memory structures that remain allocated even though they should have been freed. This creates a scenario where each failed fork operation consumes additional memory without proper cleanup, gradually depleting available system resources.
The operational impact of this vulnerability is severe, as it enables local users to perform a denial of service attack through memory exhaustion. Attackers can repeatedly invoke the clone system call with CLONE_VM flag, triggering the flawed error handling path and causing progressive memory consumption. Since the memory leak occurs within kernel space, it affects the entire system's memory availability rather than just individual processes, potentially leading to system instability, application crashes, or complete system hang. The vulnerability is particularly dangerous because it requires minimal privileges to exploit, as local users can trigger the condition without requiring special permissions or root access, making it a significant threat in multi-user environments where users might attempt to exhaust system resources.
The vulnerability aligns with CWE-401, which specifically addresses improper handling of memory allocation failures and resource leaks in software systems. From an adversarial perspective, this flaw maps to ATT&CK technique T1499.001, which involves resource exhaustion attacks targeting system memory. The exploitability is enhanced by the fact that the clone system call with CLONE_VM flag is commonly used in legitimate system operations, making the vulnerability harder to detect in normal system monitoring. The memory leak is particularly insidious because it accumulates over time, meaning that a single malicious user can cause progressive degradation of system performance rather than an immediate crash.
Mitigation strategies for this vulnerability require immediate kernel updates to versions 2.4.26 or 2.6.6 and later, which contain the necessary fixes to properly decrement the mm_count counter in error conditions. System administrators should also implement monitoring for unusual memory usage patterns and consider implementing resource limits on process creation to prevent exploitation. Additionally, security hardening measures such as disabling unnecessary system calls and implementing proper access controls can reduce the attack surface. The fix implemented in affected kernel versions ensures proper cleanup of memory management structures regardless of whether the fork operation succeeds or fails, thereby preventing the accumulation of leaked memory resources that would otherwise lead to system instability.