CVE-2025-21855 is a high-severity use-after-free vulnerability in the Linux kernel's ibmvnic network driver, which handles virtual network interfaces for IBM Power Systems virtual I/O servers (VIOS). The flaw arises because the driver references the socket buffer (skb) memory after it has been sent to the VIOS. Specifically, after flushing the transmit buffer to VIOS, the driver increments the tx_bytes statistic by reading skb->len. However, this access is unsafe because the VIOS can trigger an interrupt at any time to free the skb memory, leading to a race condition between reading skb->len and the skb being freed. This results in a use-after-free condition, which is detected by Kernel Address Sanitizer (KASAN) as a slab-use-after-free error. The vulnerability is rooted in improper memory handling and synchronization in the ibmvnic_xmit function. Exploiting this flaw could allow an attacker with limited privileges (local user with low privileges) to cause a kernel crash (denial of service) or potentially execute arbitrary code with kernel privileges, impacting confidentiality, integrity, and availability. The vulnerability does not require user interaction but does require local access with some privileges. The CVSS v3.1 score is 7.8 (high), reflecting the significant impact and moderate attack complexity. The vulnerability affects specific Linux kernel versions identified by commit hashes, primarily those running on IBM Power Systems using the ibmvnic driver. No known exploits are currently reported in the wild, but the risk remains due to the severity and kernel-level impact.

For European organizations, especially those operating IBM Power Systems infrastructure or using Linux kernels with the ibmvnic driver, this vulnerability poses a serious risk. Exploitation can lead to kernel crashes causing service outages, impacting availability of critical systems. More severe exploitation could allow privilege escalation to kernel level, compromising system integrity and confidentiality, potentially exposing sensitive data or allowing persistent unauthorized access. Industries relying on IBM Power Systems for enterprise applications, such as finance, manufacturing, and government sectors, could face operational disruptions and data breaches. Given the kernel-level nature, remediation may require downtime for patching, affecting business continuity. Additionally, organizations with compliance obligations under GDPR must consider the risk of data exposure and incident reporting requirements. The lack of known exploits reduces immediate risk but does not eliminate the threat, especially as attackers may develop exploits post-disclosure.

1. Immediate application of official Linux kernel patches that address CVE-2025-21855 is critical. Organizations should track vendor advisories and update affected kernels promptly. 2. For environments where patching is delayed, consider disabling or isolating the ibmvnic network interface if feasible to reduce exposure. 3. Implement strict access controls to limit local user privileges, minimizing the number of users who can trigger the vulnerability. 4. Employ kernel hardening techniques such as Kernel Address Sanitizer (KASAN) in testing environments to detect similar issues proactively. 5. Monitor system logs and kernel crash reports for signs of exploitation attempts or instability related to ibmvnic. 6. Coordinate with IBM and Linux distribution vendors for guidance on secure configurations and updates. 7. Plan for maintenance windows to apply patches with minimal disruption, including backups and rollback procedures. 8. Conduct security awareness for system administrators about the risks of local privilege vulnerabilities and the importance of timely patching.