CVE-2021-46910 is a vulnerability identified in the Linux kernel specifically affecting ARM architectures when the kernel is built with the CONFIG_DEBUG_KMAP_LOCAL option enabled and NR_CPUS set to 32 or higher. The vulnerability arises from the debugging code for the kmap_local() function, which doubles the number of per-CPU fixmap slots to use half as guard regions. This causes the fixmap region to grow downward beyond its reserved window if the number of CPUs is large, leading to a collision with the newly added virtual Device Tree (DT) mapping located just below the fixmap region. This overlap results in the kernel misinterpreting block entries below the fixmap region as fixmap table entries and attempting to dereference them using a phys-to-virt translation valid only for low memory. This misinterpretation leads to kernel paging faults and internal errors, causing system crashes (kernel oops). The issue manifests notably during EFI boot on affected kernels, resulting in cryptic kernel panic messages and system instability. The fix implemented limits NR_CPUS to 16 when CONFIG_DEBUG_KMAP_LOCAL is enabled and corrects the BUILD_BUG_ON() check to properly detect when the fixmap region grows below its start address rather than above its end address. This vulnerability is specific to ARM-based Linux systems with particular kernel configurations and does not affect all Linux deployments. No known exploits are reported in the wild at this time.

For European organizations, the impact of CVE-2021-46910 depends largely on their use of ARM-based Linux systems with debugging enabled for kmap_local and high CPU counts (NR_CPUS >= 32). Such configurations are more common in development, embedded, or specialized ARM server environments rather than typical x86 enterprise servers. The vulnerability can cause system crashes and instability, potentially leading to denial of service conditions. This could disrupt critical services, especially in sectors relying on ARM-based infrastructure such as telecommunications, IoT deployments, and edge computing. Confidentiality and integrity impacts are minimal as the vulnerability primarily causes availability issues through kernel panics. However, repeated crashes could lead to operational downtime and increased maintenance costs. Since exploitation requires specific kernel build configurations and hardware architectures, the threat is limited in scope but should not be ignored in environments where these conditions apply.

European organizations should audit their Linux kernel configurations on ARM-based systems to determine if CONFIG_DEBUG_KMAP_LOCAL is enabled and if NR_CPUS is set to 32 or higher. For production systems, it is advisable to disable CONFIG_DEBUG_KMAP_LOCAL unless actively debugging kernel memory mappings. If debugging is necessary, limit NR_CPUS to 16 as per the patch recommendations. Organizations should apply the latest Linux kernel updates that include the fix for CVE-2021-46910 to ensure the BUILD_BUG_ON() check correctly prevents the fixmap region overlap. Additionally, thorough testing of kernel updates in staging environments is recommended to verify stability before deployment. Monitoring kernel logs for paging faults or oops messages related to fixmap or kmap_local can help detect potential issues early. For embedded or ARM-based devices, coordinate with hardware vendors to ensure firmware and kernel patches are applied promptly. Avoid running kernels with debug options enabled in production unless absolutely necessary.