CVE-2025-37929 is a vulnerability identified in the Linux kernel specifically affecting the arm64 architecture. The issue arises from a missing sentinel entry in the arrays used for the Spectre-BHB (Branch History Buffer) mitigation. The Spectre-BHB vulnerability is a speculative execution side-channel attack targeting certain ARM CPUs. To mitigate this, the Linux kernel maintains lists of CPU MIDRs (Model ID Registers) that require specific workarounds. In commit a5951389e58d, new ARM cores were added to the Spectre-BHB workaround lists, including new arrays with updated 'k' values necessary for effective mitigation. However, these new arrays omitted the sentinel entries that mark the end of the list. Consequently, the function is_midr_in_range_list(), which checks if a CPU MIDR is within these lists, can walk off the end of the array when no match is found. This leads to undefined behavior and, when the Undefined Behavior Sanitizer (UBSAN) is enabled, causes a kernel crash during boot. The crash manifests as a kernel panic with an internal error indicating a fatal exception on aarch64 architecture. The problem is particularly triggered when is_midr_in_range_list() is inlined, a behavior more common prior to a later commit that changed the function's export status. The fix involves adding the missing sentinel entries to the arrays to prevent out-of-bounds access. This vulnerability does not appear to be exploitable for code execution or privilege escalation but results in denial of service due to kernel panic during system startup on affected ARM64 Linux systems with UBSAN enabled. No known exploits are reported in the wild as of the publication date.

For European organizations running Linux on ARM64 platforms, especially those using kernels compiled with UBSAN or similar sanitizers, this vulnerability can cause system instability and denial of service during boot. This is particularly relevant for embedded systems, ARM-based servers, and edge devices that rely on Linux kernels with these specific commits. The inability to boot can disrupt critical services, leading to operational downtime. While the vulnerability does not directly compromise confidentiality or integrity, the availability impact can be significant for infrastructure relying on ARM64 Linux systems. Organizations deploying ARM64-based Linux in cloud environments, telecommunications, IoT, or industrial control systems may face service interruptions. The impact is mitigated if UBSAN is not enabled or if systems do not use the affected kernel versions. However, the presence of this bug in official Linux kernel releases means that unpatched systems could encounter boot failures after kernel updates or during system initialization, affecting maintenance and deployment processes.

1. Apply the official Linux kernel patch that adds the missing sentinel entries to the Spectre-BHB MIDR arrays as soon as it becomes available. Monitor Linux kernel mailing lists and vendor advisories for updated stable kernel releases addressing CVE-2025-37929. 2. Avoid using kernel builds with UBSAN enabled in production environments unless necessary for debugging, as UBSAN triggers the crash. 3. For systems that must use UBSAN-enabled kernels, consider temporarily reverting to earlier kernel versions prior to the problematic commit or disabling the Spectre-BHB workaround if acceptable and safe. 4. Implement robust testing and validation of kernel updates in staging environments, particularly for ARM64 platforms, to detect boot-time crashes before deployment. 5. Maintain backup and recovery procedures to restore systems that fail to boot due to this issue. 6. Engage with hardware and Linux distribution vendors to ensure timely patch distribution and support for affected ARM64 platforms. 7. For embedded and IoT devices, coordinate firmware and kernel updates carefully to avoid bricking devices in the field.