CVE-2021-47116 in Linux
Summary
by MITRE • 03/15/2024
In the Linux kernel, the following vulnerability has been resolved:
ext4: fix memory leak in ext4_mb_init_backend on error path.
Fix a memory leak discovered by syzbot when a file system is corrupted with an illegally large s_log_groups_per_flex.
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Analysis
by VulDB Data Team • 01/07/2025
The vulnerability identified as CVE-2021-47116 represents a memory leak flaw within the Linux kernel's ext4 file system implementation that occurs during error handling scenarios. This issue manifests specifically when the kernel attempts to initialize the backend memory management structures for ext4 file systems, particularly when encountering corrupted file system metadata. The memory leak occurs in the ext4_mb_init_backend function, which is responsible for managing memory allocation for the ext4 file system's block allocation backend. The vulnerability was discovered through automated fuzzing conducted by syzbot, a sophisticated kernel fuzzer that systematically tests kernel code paths for potential issues. When a file system is corrupted with an artificially large s_log_groups_per_flex value, the kernel's error handling path fails to properly release allocated memory resources, leading to a gradual accumulation of unreleased memory within the kernel's address space. This type of memory leak represents a denial of service vulnerability that could potentially degrade system performance or lead to system instability if left unaddressed.
The technical root cause of this vulnerability stems from improper memory management within the ext4 file system's error handling code path. The s_log_groups_per_flex parameter controls the number of flexible group structures within the ext4 file system, and when this parameter is set to an illegally large value, it triggers a condition where the kernel's memory allocation functions are called but not properly freed during error recovery. The flaw exists in the ext4_mb_init_backend function where memory is allocated for internal data structures but the cleanup code path fails to release these allocations when an error condition is encountered. This type of memory leak falls under the CWE-401 category of "Improper Release of Memory Before Removing Last Reference" and represents a classic case of resource management failure in kernel space code. The vulnerability demonstrates a fundamental flaw in defensive programming practices where error recovery paths do not properly account for all allocated resources, leading to memory exhaustion over time. The error path in question specifically handles cases where the ext4 file system initialization fails due to corrupted metadata, but the cleanup process does not adequately address all memory allocations that occurred during the initialization attempt.
The operational impact of CVE-2021-47116 extends beyond simple memory consumption issues as it represents a potential vector for denial of service attacks against systems running affected Linux kernels. When exploited, this vulnerability allows an attacker with access to modify file system metadata to gradually consume system memory resources, potentially leading to system instability, application crashes, or complete system hangs. The memory leak is particularly concerning in environments where ext4 file systems are heavily utilized or where automated processes might repeatedly attempt to access corrupted file systems. Systems with limited memory resources are especially vulnerable to this attack vector, as the gradual accumulation of leaked memory can quickly lead to system resource exhaustion. The vulnerability affects any Linux system running a kernel version prior to the fix, including servers, desktop systems, and embedded devices that utilize ext4 file systems. The impact is amplified in virtualized environments where memory pressure can affect multiple virtual machines sharing the same host resources, creating cascading failures that could compromise entire infrastructure deployments. This vulnerability also has implications for cloud computing platforms and containerized environments where file system corruption could be induced through various attack vectors, including malicious file uploads or compromised applications.
Mitigation strategies for CVE-2021-47116 focus on immediate kernel updates and system hardening measures to prevent exploitation. The primary and most effective mitigation is to apply the kernel patch that resolves the memory leak by ensuring proper cleanup of allocated memory during error handling scenarios. System administrators should prioritize updating to kernel versions that include the fix, typically those released after the vulnerability disclosure date. Additional defensive measures include implementing robust file system monitoring and integrity checking mechanisms that can detect and alert on corrupted file system metadata before it can trigger the vulnerable code path. The use of automated file system checking tools like e2fsck in read-only mode can help identify and prevent corrupted file systems from being mounted, thereby avoiding the error condition that triggers the memory leak. Organizations should also consider implementing memory monitoring solutions that can detect unusual memory consumption patterns and alert on potential memory leak exploitation attempts. Network segmentation and access controls should be implemented to limit potential attack vectors that could lead to file system corruption, particularly in environments where user uploads or external data sources are involved. The fix addresses the underlying issue through proper resource management and ensures that all allocated memory is released regardless of the execution path taken during error conditions, aligning with best practices for kernel memory management and defensive programming techniques recommended in various security frameworks and standards.