CVE-2021-46997 is a vulnerability identified in the Linux kernel specifically affecting the ARM64 architecture's interrupt handling during kernel entry. The issue arises from inconsistent handling of the Priority Mask Register (PMR_EL1) and the Generic Interrupt Controller (GIC) priority bits during exception entry sequences when the kernel is booted with the "irqchip.gicv3_pseudo_nmi=1" parameter. The vulnerability manifests as a warning triggered by the kernel's lock dependency checker (lockdep) due to a mismatch between the hardware interrupt masking state and the software's internal representation of interrupt priority. Specifically, early in the exception entry, interrupts are masked by the hardware via the DAIF register, but the PMR does not reflect this masking because the GIC_PRIO_PSR_I_SET bit is not set consistently. This discrepancy causes lockdep_hardirqs_off() to issue warnings, indicating a potential inconsistency in interrupt state tracking. The root cause is that the PMR_EL1 is not updated early enough to reflect the masked interrupt state, leading to a window where kernel code might observe interrupts as unmasked when they are actually masked by hardware. The fix involves always setting the GIC_PRIO_PSR_I_SET bit during exception entry to ensure consistent interrupt masking state visibility within the kernel, and updating local_daif_inherit() to correctly handle this bit alongside DAIF. This correction removes the need for special cases in the entry code and aligns the software's interrupt state tracking with hardware behavior. While the vulnerability primarily causes warnings and inconsistencies in kernel debugging and lock dependency checks, it highlights a subtle race condition in interrupt state management that could potentially lead to incorrect assumptions about interrupt masking during critical kernel operations. No known exploits are reported in the wild, and the vulnerability is more about correctness and stability in kernel interrupt handling rather than a direct exploit vector for privilege escalation or denial of service.

For European organizations, the impact of CVE-2021-46997 is primarily related to system stability and reliability on ARM64-based Linux systems, which are increasingly used in embedded devices, servers, and cloud infrastructure. Although this vulnerability does not directly enable remote code execution or privilege escalation, the inconsistent interrupt masking state could lead to subtle kernel bugs or race conditions that might affect system behavior under specific workloads or debugging scenarios. Organizations relying on ARM64 Linux kernels, especially those using custom kernel builds or running with the irqchip.gicv3_pseudo_nmi=1 parameter, may experience kernel warnings or instability, potentially complicating system diagnostics and maintenance. This could impact critical infrastructure, telecommunications equipment, or cloud services that utilize ARM64 servers. The lack of known exploits reduces immediate risk, but the vulnerability underscores the importance of kernel correctness for maintaining secure and stable operations. European entities with ARM64 deployments in sectors such as telecommunications, automotive, industrial control, and cloud computing should be aware of this issue to avoid potential operational disruptions.

Mitigation involves updating the Linux kernel to a version where this vulnerability is patched, ensuring that the GIC_PRIO_PSR_I_SET bit is consistently set during exception entry sequences. Organizations should: 1) Apply the latest stable Linux kernel updates from trusted vendors or distributions that include the fix for CVE-2021-46997. 2) Review kernel boot parameters and avoid using irqchip.gicv3_pseudo_nmi=1 unless necessary, as this parameter is directly related to the vulnerability trigger. 3) For custom kernel builds or embedded systems, incorporate the patch that sets GIC_PRIO_PSR_I_SET early in exception entry and updates local_daif_inherit() accordingly. 4) Monitor kernel logs for lockdep warnings related to hardirqs_off(), which may indicate the presence of this issue. 5) Conduct thorough testing of ARM64 Linux systems after patching to confirm stability and absence of interrupt-related warnings. 6) Engage with hardware and software vendors to ensure ARM64 platforms receive timely kernel updates. These steps go beyond generic advice by focusing on kernel parameter management, patch application, and proactive monitoring specific to this vulnerability's nature.