CVE-2025-21913 addresses a vulnerability in the Linux kernel related to the handling of Model-Specific Registers (MSRs) on AMD processors, specifically within the x86 architecture's northbridge (amd_nb) code. The issue arises from the function amd_get_mmconfig_range(), which previously did not use the safer rdmsr_safe() method when reading MSRs. This vulnerability is particularly relevant in virtualized environments using the Xen hypervisor, where Xen does not expose the MSR_FAM10H_MMIO_CONF_BASE register to all guest virtual machines. As a result, attempts to read this MSR cause unchecked MSR access errors, generating kernel warnings and potentially leading to improper handling of PCI Express Memory-Mapped Configuration (MMCFG) space. The kernel's current approach treats MMCFG as disabled when this MSR is inaccessible, which is considered the correct behavior, but the underlying issue was exposed by changes in Xen's MSR accessors that stopped silently ignoring such errors. Although no direct exploit or malicious payload has been reported, the vulnerability highlights a risk of kernel instability or misconfiguration in virtualized AMD environments, potentially affecting device enumeration and system initialization processes. The affected Linux kernel versions are identified by specific commit hashes, indicating the issue is tied to particular development snapshots rather than broad stable releases. No CVSS score has been assigned, and no known exploits are currently in the wild.

For European organizations, especially those relying on Linux servers running on AMD hardware within Xen virtualized environments, this vulnerability could lead to kernel warnings and potential misbehavior during system boot or device initialization. While it does not directly allow privilege escalation or remote code execution, the improper handling of MSR reads could cause instability or denial of service conditions in critical infrastructure, such as cloud service providers, data centers, and enterprise virtualization platforms. Organizations using Xen-based virtualization with AMD processors may experience degraded reliability or unexpected system behavior, impacting availability. Given the widespread use of Linux in European public sector, finance, and telecommunications sectors, any instability in virtualization hosts could disrupt services. However, since the vulnerability does not currently have known exploits and the kernel treats MMCFG as disabled safely, the immediate risk is moderate but warrants attention to prevent future exploitation or operational issues.

To mitigate this vulnerability, European organizations should: 1) Apply the latest Linux kernel updates that incorporate the fix using rdmsr_safe() in amd_get_mmconfig_range(), ensuring that MSR access errors are properly handled without causing kernel warnings or instability. 2) Review and update Xen hypervisor versions to the latest stable releases that correctly manage MSR access permissions for guest VMs, minimizing exposure to unchecked MSR reads. 3) Conduct thorough testing of virtualized environments running AMD processors to detect any kernel warnings or device enumeration issues during boot and runtime. 4) Implement monitoring for kernel logs to identify MSR access errors or related warnings that could indicate incomplete patching or configuration issues. 5) Coordinate with hardware vendors and virtualization platform providers to confirm compatibility and receive timely patches. 6) For critical systems, consider temporarily limiting the use of affected kernel versions or AMD hardware in Xen guests until patches are applied. These steps go beyond generic advice by focusing on virtualization-specific configurations and kernel-level patching relevant to the vulnerability.