CVE-2025-21984 is a vulnerability identified in the Linux kernel's memory management subsystem, specifically related to the userfaultfd_move() function. This function is responsible for handling page table entries (PTEs) during memory migration operations. The vulnerability arises from an incorrect handling of PTEs that are swap entries but still reference folios present in the swap cache. The flawed logic assumes that if a PTE is a swap entry, it can be simply copied to the destination address without updating the folio's index. However, since the folio remains in the swap cache, this leads to a race condition between the conversion of PTEs to swap entries and the clearing of the swap cache. If userfaultfd_move() executes during this window, the destination PTE points to a folio whose index does not match the destination virtual memory area (VMA). This mismatch triggers a kernel BUG during operations such as add_rmap, which relies on consistent page offsets. The bug manifests as a kernel panic or oops, causing system instability or crashes. The issue is exacerbated when Kernel Samepage Merging (KSM) is disabled, affecting both small and large folios. The vulnerability stems from a fundamental race condition in the kernel's memory management code, potentially leading to denial of service (DoS) via kernel crashes. No evidence currently suggests privilege escalation or arbitrary code execution, but the kernel panic impact is severe for system availability. The vulnerability affects Linux kernel versions identified by the commit hash adef440691bab824e39c1b17382322d195e1fab0 and similar builds. No CVSS score has been assigned yet, and no known exploits are reported in the wild as of the publication date (April 1, 2025).

For European organizations, the impact of CVE-2025-21984 primarily concerns system availability and reliability. Linux is widely deployed across European enterprises, government agencies, cloud providers, and critical infrastructure sectors. A kernel BUG leading to system crashes can cause denial of service, disrupting business operations, data processing, and service delivery. Systems running workloads with frequent memory migration or userfaultfd usage—such as virtualized environments, container orchestration platforms, and high-performance computing clusters—are particularly at risk. The instability may also complicate incident response and recovery efforts. While no direct data confidentiality or integrity compromise is indicated, the operational disruption could indirectly affect compliance with regulations like GDPR if services become unavailable or data processing is interrupted. Furthermore, critical sectors such as finance, healthcare, and energy that rely on Linux-based systems for core operations could face significant operational risk. The absence of known exploits reduces immediate threat but does not eliminate the risk of future weaponization by attackers targeting availability.

To mitigate CVE-2025-21984, European organizations should: 1) Apply the official Linux kernel patches addressing this vulnerability as soon as they become available, ensuring that all affected systems are updated promptly. 2) Prioritize patching for systems running kernel versions identified as vulnerable, especially those involved in virtualization, containerization, or memory-intensive workloads. 3) Enable Kernel Samepage Merging (KSM) where feasible, as the vulnerability impact is more severe when KSM is disabled; enabling KSM may reduce the likelihood of triggering the kernel BUG. 4) Implement robust monitoring of kernel logs and system stability to detect early signs of kernel panics or oops events related to memory management. 5) For critical production environments, consider staged rollouts and thorough testing of kernel updates to avoid unintended disruptions. 6) Maintain comprehensive backups and disaster recovery plans to minimize downtime in case of crashes. 7) Limit user access to privileged operations that could trigger userfaultfd_move() calls, reducing the attack surface. 8) Collaborate with Linux distribution vendors and security communities to stay informed about patch releases and exploit developments.