CVE-2025-21655 is a vulnerability identified in the Linux kernel, specifically related to the io_uring subsystem's eventfd handling. The issue arises in the function io_eventfd_signal(), which is responsible for signaling events via eventfd within the io_uring asynchronous I/O interface. The vulnerability concerns the improper management of reference counting and memory freeing in the context of Read-Copy-Update (RCU) callbacks. When io_eventfd_do_signal() is invoked from an RCU callback, it decrements the reference count of the io_ev_fd object and, if the count reaches zero, directly calls io_eventfd_free() to free the object. This direct freeing is incorrect because it bypasses the necessary deferral of freeing until after another RCU grace period, which is critical to ensure safe memory reclamation in concurrent environments. The correct approach, as fixed, is to call io_eventfd_put(), which properly defers the freeing operation until after the next RCU grace period, preventing potential use-after-free or race conditions. This vulnerability could lead to memory corruption or use-after-free scenarios, potentially allowing an attacker to cause kernel crashes or escalate privileges if exploited. However, no known exploits are currently reported in the wild, and the vulnerability requires detailed knowledge of kernel internals and likely local access to trigger. The affected versions are specific Linux kernel commits identified by their hashes, indicating this is a vulnerability in recent or development versions of the kernel. No CVSS score has been assigned yet, and no public exploit code is available at this time.

For European organizations, the impact of CVE-2025-21655 depends largely on their use of Linux systems, particularly those running recent or development kernel versions that include the vulnerable io_uring implementation. Organizations relying on Linux servers for critical infrastructure, cloud services, or embedded systems could face risks of system instability or potential privilege escalation if the vulnerability is exploited. This could lead to denial of service due to kernel panics or, in worst cases, unauthorized access to sensitive data or system control. Given the kernel-level nature of the vulnerability, successful exploitation could compromise the confidentiality, integrity, and availability of affected systems. However, the complexity of exploitation and the requirement for local code execution or privileged access reduce the likelihood of widespread impact. Still, sectors such as finance, telecommunications, and government agencies in Europe that depend heavily on Linux infrastructure should consider this vulnerability seriously, as kernel compromises can have cascading effects on service availability and data security.

European organizations should promptly update their Linux kernels to versions where this vulnerability is patched. Since the vulnerability relates to specific kernel commits, organizations should track official Linux kernel releases and apply updates from trusted sources or vendors. For environments using custom or development kernels, recompiling the kernel with the patched io_uring code is essential. Additionally, organizations should restrict local access to critical Linux systems to trusted users only, minimizing the risk of local exploitation. Employing kernel hardening techniques such as SELinux, AppArmor, or seccomp can provide additional layers of defense. Monitoring system logs for unusual kernel errors or crashes related to io_uring or eventfd can help detect potential exploitation attempts. Finally, organizations should maintain robust incident response plans to quickly address any kernel-level compromises.