CVE-2025-21881 is a vulnerability in the Linux kernel related to the uprobe subsystem, specifically in the function uprobe_write_opcode(). The issue arises from improper handling of the shared zero page (zero folio) during memory page reference counting. The vulnerability was discovered through syzkaller fuzz testing, which triggered kernel crashes due to bad page states and incorrect RSS (resident set size) counters. The root cause is that when a zero page frame number (pfn) is set in the page table entry (PTE) without incrementing the RSS count in mfill_atomic_pte_zeropage(), the zero folio's reference count is not increased accordingly. Subsequently, uprobe_write_opcode() calls __replace_page(), which unconditionally decrements the RSS count and the old folio's reference count. This leads to two critical bugs: (1) an incorrect RSS count causing "Bad rss-count" errors during process exit, and (2) premature freeing of the reserved zero folio when its reference count reaches zero, causing "Bad page state" errors. Additionally, a rare warning related to zero folio detection in __replace_page() could be triggered. The vulnerability is triggered when uprobe hits the zero folio, which is a rare case, but the fix involves rejecting zero old folios immediately after get_user_page_vma_remote() to prevent these errors. The vulnerability affects Linux kernel versions identified by the given commit hashes and was publicly disclosed on March 27, 2025. No known exploits are currently reported in the wild. This vulnerability impacts kernel stability and memory management integrity, potentially leading to system crashes or denial of service (DoS) conditions when exploited.

For European organizations, the impact of CVE-2025-21881 primarily involves system stability and availability risks. Since the vulnerability can cause kernel crashes and memory corruption, affected Linux systems may experience unexpected reboots or denial of service, disrupting critical services and operations. Organizations relying on Linux servers for web hosting, cloud infrastructure, container orchestration, or embedded systems could face operational downtime. The vulnerability does not directly indicate privilege escalation or data confidentiality breaches; however, system crashes could indirectly affect data integrity and availability. Given the widespread use of Linux in European enterprises, public sector, and telecommunications, the disruption potential is significant, especially in environments where uptime and reliability are critical. The lack of known exploits reduces immediate risk, but the complexity of the vulnerability and its kernel-level nature mean that sophisticated attackers or accidental triggers could cause impactful outages. Additionally, the vulnerability affects kernel memory management internals, which are foundational to system security and stability, so unpatched systems remain at risk of instability and potential exploitation in the future.

To mitigate CVE-2025-21881, European organizations should: 1) Apply the official Linux kernel patches that address the zero folio reference counting and RSS count issues as soon as they become available from trusted Linux distribution vendors or the Linux kernel mainline. 2) Prioritize patching on systems running kernel versions identified as vulnerable, especially those used in production, cloud, or critical infrastructure environments. 3) Implement kernel live patching solutions where possible to reduce downtime during patch deployment. 4) Monitor kernel logs and system crash reports for symptoms related to bad page states or rss-count errors, which may indicate attempts to trigger this vulnerability. 5) Restrict unprivileged user access to debugging and tracing facilities that use uprobes, as exploitation requires triggering uprobe_write_opcode() paths. 6) Harden container and virtualization environments by limiting capabilities and isolating workloads to reduce the attack surface. 7) Maintain robust backup and recovery procedures to minimize impact from potential system crashes. 8) Engage with Linux distribution security advisories and subscribe to vulnerability notifications to stay informed about updates and exploit developments related to this vulnerability.