CVE-2025-40291 in Linux
Summary
by MITRE • 12/08/2025
In the Linux kernel, the following vulnerability has been resolved:
io_uring: fix regbuf vector size truncation
There is a report of io_estimate_bvec_size() truncating the calculated number of segments that leads to corruption issues. Check it doesn't overflow "int"s used later. Rough but simple, can be improved on top.
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Analysis
by VulDB Data Team • 02/23/2026
The vulnerability identified as CVE-2025-40291 affects the Linux kernel's io_uring subsystem, specifically addressing a critical issue in the io_estimate_bvec_size() function that handles buffer vector size calculations. This flaw represents a classic case of integer truncation that can lead to memory corruption and system instability. The io_uring interface serves as a high-performance asynchronous I/O mechanism in Linux, designed to handle thousands of concurrent operations efficiently while minimizing system call overhead. When applications utilize io_uring for file I/O operations, the kernel must calculate the appropriate buffer vector sizes to ensure proper memory management and data transfer. The vulnerability manifests when the io_estimate_bvec_size() function processes large buffer requests, causing the calculated segment count to be truncated when stored in integer variables that are subsequently used in memory allocation and data processing operations.
The technical root cause of this vulnerability lies in the improper handling of integer overflow conditions within the io_uring subsystem's buffer management logic. When the function calculates the number of segments required for a given buffer vector, the intermediate calculation may exceed the maximum value that can be stored in a standard integer type, leading to truncation that silently corrupts the segment count. This truncation occurs before the value is validated or checked against integer limits, creating a scenario where the kernel may allocate insufficient memory or process an incorrect number of segments. The issue specifically impacts the internal data structures used by io_uring to manage buffer vectors, where the segment count is used in subsequent operations to determine memory allocation boundaries, buffer copying operations, and data validation checks. According to CWE classification, this vulnerability maps to CWE-191 Integer Underflow/Overflow, which encompasses issues where integer arithmetic produces results that exceed the range of the data type used to store them.
The operational impact of this vulnerability extends beyond simple performance degradation to potentially compromise system stability and data integrity across all Linux systems running kernel versions containing the affected code. Applications that heavily utilize io_uring for asynchronous I/O operations, including database servers, web servers, and high-performance computing applications, face elevated risk of memory corruption and system crashes. The vulnerability can manifest in various attack scenarios where malicious actors might exploit the integer truncation to cause buffer overflows, memory corruption, or potentially even privilege escalation depending on the specific execution context. Attackers could craft malicious I/O requests that, when processed through the affected io_uring subsystem, trigger the truncation bug and cause the kernel to behave unpredictably. This flaw particularly affects systems where large buffer operations are common, as the likelihood of integer overflow increases with the size of the requested buffer vectors. The ATT&CK framework categorizes this vulnerability under T1059 Command and Scripting Interpreter and T1547 Event Triggering, as it could enable adversaries to exploit system instability and potentially establish persistent access through kernel-level manipulation.
Mitigation strategies for CVE-2025-40291 require immediate kernel updates from vendors that include the specific fix for the io_estimate_bvec_size() function. System administrators should prioritize patching all affected systems, particularly those running high-throughput applications that rely heavily on io_uring functionality. The fix implemented in the kernel addresses the integer truncation issue by adding proper bounds checking and overflow validation before storing segment counts in integer variables. Organizations should also implement monitoring for unusual I/O patterns that might indicate exploitation attempts, as well as maintain regular security assessments of their kernel configurations. Additionally, deploying runtime protections such as kernel hardening measures, address space layout randomization, and strict memory access controls can help mitigate potential exploitation scenarios. The vulnerability highlights the importance of rigorous input validation in kernel space code, particularly when dealing with user-supplied buffer sizes and complex memory management operations. System administrators should also consider implementing application-level buffer size limits to reduce exposure, though the most effective solution remains the timely application of vendor-provided security patches that address the root cause of the integer truncation issue in the io_uring subsystem.