CVE-2025-39963 in Linux
Summary
by MITRE • 10/09/2025
In the Linux kernel, the following vulnerability has been resolved:
io_uring: fix incorrect io_kiocb reference in io_link_skb
In io_link_skb function, there is a bug where prev_notif is incorrectly assigned using 'nd' instead of 'prev_nd'. This causes the context validation check to compare the current notification with itself instead of comparing it with the previous notification.
Fix by using the correct prev_nd parameter when obtaining prev_notif.
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Analysis
by VulDB Data Team • 02/03/2026
The vulnerability identified as CVE-2025-39963 resides within the Linux kernel's io_uring subsystem, a high-performance asynchronous I/O framework designed to handle thousands of I/O operations efficiently. This subsystem is critical for modern Linux systems as it provides a scalable interface for applications to perform asynchronous file and network I/O operations. The flaw manifests in the io_link_skb function which is responsible for managing linked I/O operations and maintaining proper notification contexts within the io_uring framework. This particular vulnerability represents a subtle but significant error in context validation logic that could potentially compromise the integrity of asynchronous I/O operations.
The technical flaw occurs in the io_link_skb function where a variable naming error leads to incorrect reference handling during notification context validation. Specifically, the code incorrectly assigns the prev_notif variable using the nd parameter instead of the prev_nd parameter, creating a scenario where the system validates a notification against itself rather than against its predecessor in the notification chain. This fundamental error in reference management stems from a simple but critical typo that affects how the io_uring subsystem maintains state consistency across linked I/O operations. The bug essentially creates a validation loop where the system compares a notification with its own context, potentially leading to incorrect decision-making in I/O operation linking and notification handling.
The operational impact of this vulnerability extends beyond simple functional degradation as it affects the core reliability of asynchronous I/O operations within Linux systems. When the context validation fails due to comparing a notification with itself, it can cause unpredictable behavior in I/O operation chaining, potentially leading to missed notifications, incorrect operation linking, or even system instability under high I/O load conditions. This vulnerability is particularly concerning because io_uring is extensively used in high-performance applications including web servers, database systems, and containerized environments where reliable asynchronous I/O is crucial. The flaw could manifest as subtle race conditions or data integrity issues that are difficult to reproduce and debug, making it a particularly dangerous vulnerability in production systems where I/O reliability is paramount.
The fix for CVE-2025-39963 involves a straightforward correction to the parameter usage within the io_link_skb function, replacing the incorrect nd parameter with the proper prev_nd parameter when retrieving the prev_notif context. This change ensures that notification validation properly compares the current notification with its predecessor rather than with itself, restoring the intended validation logic. From a cybersecurity perspective, this vulnerability aligns with CWE-457: Use of Uninitialized Variable, as the incorrect reference effectively creates an uninitialized validation context that leads to unpredictable behavior. The fix addresses the underlying issue by ensuring proper state management and context validation within the io_uring subsystem, thereby maintaining the integrity of asynchronous I/O operation chains. Organizations should prioritize applying this patch as it resolves a potential vector for denial of service attacks or data corruption in systems relying on io_uring for high-performance I/O operations, particularly in cloud environments and container orchestration platforms where such performance-critical subsystems are extensively utilized.