CVE-2025-21950 is a vulnerability identified in the Linux kernel, specifically within the virtual drivers component related to the ACRN hypervisor subsystem (drivers/virt/acrn/hsm). The issue arises in the pmcmd_ioctl function, where three memory objects allocated using kmalloc are initialized by the hcall_get_cpu_state function and subsequently copied to user space. However, the initialization is incomplete because the underlying initializer, acrn_hypercall2, does not zero out all bytes of the allocated memory. This results in uninitialized bytes being leaked to user space, potentially exposing sensitive kernel memory contents. The root cause is the use of kmalloc without zeroing the allocated memory, which can contain residual data from previous kernel operations. The fix involves switching to kzalloc, which allocates and zeroes memory, thereby preventing leakage of uninitialized data. Although this vulnerability does not allow direct code execution or privilege escalation, it constitutes an information leak that could aid attackers in bypassing security mechanisms such as kernel address space layout randomization (KASLR) or in gathering sensitive information from kernel memory. The vulnerability affects Linux kernel versions identified by the commit hash 3d679d5aec648f50e645702929890b9611998a0b and likely other versions containing the same code pattern. No known exploits are reported in the wild as of the publication date (April 1, 2025). No CVSS score has been assigned yet.

For European organizations, the primary impact of CVE-2025-21950 is the potential exposure of sensitive kernel memory contents to unprivileged user space processes. This information leakage can facilitate further attacks by providing attackers with insights into kernel memory layout and potentially sensitive data, which could be leveraged to bypass security controls or escalate privileges in multi-tenant environments such as cloud infrastructures or virtualized environments using the ACRN hypervisor. Organizations relying on Linux-based systems, especially those using virtualization or embedded systems with ACRN, may face increased risk of targeted reconnaissance and subsequent exploitation. While the vulnerability itself does not directly compromise system integrity or availability, it weakens the security posture and could be a stepping stone for more severe attacks. This is particularly relevant for sectors with high security requirements such as finance, critical infrastructure, and government agencies within Europe.

To mitigate CVE-2025-21950, European organizations should: 1) Apply the official Linux kernel patches that replace kmalloc with kzalloc in the affected ACRN hypervisor code paths to ensure memory is zero-initialized before being copied to user space. 2) Regularly update Linux kernel versions to incorporate security fixes and monitor vendor advisories for backported patches in enterprise distributions. 3) Restrict access to the vulnerable ioctl interface (pmcmd_ioctl) to trusted users only, using appropriate access control mechanisms and SELinux/AppArmor policies to limit exposure. 4) Employ runtime security monitoring to detect unusual or unauthorized use of the pmcmd_ioctl interface. 5) In virtualized environments, isolate workloads and enforce strict tenant separation to reduce the impact of potential information leaks. 6) Conduct security audits and code reviews for custom kernel modules or drivers that interact with user space to ensure proper memory initialization practices are followed.