CVE-2025-40077 in Linux
Summary
by MITRE • 10/28/2025
In the Linux kernel, the following vulnerability has been resolved:
f2fs: fix to avoid overflow while left shift operation
Should cast type of folio->index from pgoff_t to loff_t to avoid overflow while left shift operation.
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Analysis
by VulDB Data Team • 02/15/2026
The vulnerability identified as CVE-2025-40077 resides within the Linux kernel's f2fs file system implementation, specifically addressing a critical integer overflow condition that could potentially lead to system instability or security compromise. This issue manifests during left shift operations involving file system metadata, where improper type handling creates opportunities for unexpected behavior. The f2fs file system, designed for flash storage devices and widely adopted in embedded systems and mobile platforms, processes file operations through folio structures that manage file data pages. The vulnerability occurs when the folio->index field, which represents the page offset within a file, undergoes left shift operations without proper type casting, creating conditions where the operation may exceed the maximum representable value for the data type involved.
The technical flaw stems from a type casting oversight in the f2fs subsystem where the pgoff_t type, which typically represents page offsets in file systems, is not properly converted to loff_t before undergoing left shift operations. This type mismatch creates a scenario where values that might be valid for pgoff_t could overflow or produce incorrect results when interpreted as loff_t, particularly when the left shift operation multiplies the value by powers of two. The underlying issue is fundamentally a violation of type safety principles, where the kernel's memory management and file system operations rely on precise data type handling to maintain system integrity. When the folio->index value is processed through left shift operations without proper type conversion, it can result in arithmetic overflow conditions that may lead to memory corruption or unpredictable system behavior.
The operational impact of this vulnerability extends beyond simple kernel instability, as it could enable attackers to manipulate file system operations in ways that might compromise system security or availability. In environments where f2fs is used for critical storage operations, such as embedded devices, mobile platforms, or systems with constrained memory resources, this overflow condition could potentially be exploited to cause denial of service attacks or even privilege escalation scenarios. The vulnerability affects systems running Linux kernels that incorporate the f2fs file system, particularly those that handle large files or perform extensive file system operations, as the overflow conditions become more likely with larger index values. The attack surface is significant given f2fs's adoption in various IoT devices, smartphones, and embedded systems where memory constraints and specific file system requirements make such overflow conditions more probable.
Mitigation strategies for this vulnerability require immediate kernel updates that implement the proper type casting from pgoff_t to loff_t before left shift operations are performed on folio->index values. System administrators should prioritize patching affected systems, particularly those running embedded Linux distributions or custom kernel builds that may not have received the specific fix. The recommended solution involves modifying the f2fs code to explicitly cast folio->index to loff_t type before any arithmetic operations, ensuring that the full range of values can be properly handled without overflow conditions. This fix aligns with established security practices and follows the principle of least privilege in kernel space operations, where proper type handling prevents unintended behavior that could be exploited by malicious actors. Organizations should also implement monitoring for unusual file system behavior or memory allocation patterns that might indicate exploitation attempts, as the vulnerability's effects could manifest through subtle system performance degradation or unexpected file system errors that precede more severe failures.