CVE-2021-46997: 
Linux Kernel vulnerability analysis and mitigation

In the Linux kernel, a vulnerability was discovered related to the GIC (Generic Interrupt Controller) priority masking during kernel entry on ARM64 systems. The issue affects Linux kernel versions from 5.10 up to (excluding) 5.10.38, 5.11 up to (excluding) 5.11.22, and 5.12 up to (excluding) 5.12.5. The vulnerability was discovered when booting a kernel with 'irqchip.gicv3_pseudo_nmi=1' parameter (NVD).

The vulnerability stems from inconsistent handling of the PMR (Priority Mask Register) during exception entry. Early in the entry sequence, lockdep_hardirqs_off() is called to inform lockdep that interrupts have been masked, but PMR_EL1 is not affected by exception entry. This leads to a situation where the PMR can indicate interrupts are unmasked even though they are masked by DAIF. When DEBUG_LOCKDEP is selected, this causes lockdep_hardirqs_off() to trigger a warning before GIC_PRIO_PSR_I_SET is set in any exception entry paths (Kernel Patch).

The vulnerability has a CVSS v3.1 Base Score of 5.5 (Medium) with vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H. While this issue primarily affects system stability and debugging capabilities, it could potentially lead to system instability or incorrect interrupt handling behavior (NVD).

The vulnerability requires local access and can only be triggered when booting the kernel with specific parameters (irqchip.gicv3_pseudo_nmi=1). It primarily manifests during kernel debugging scenarios with DEBUG_LOCKDEP enabled (NVD).

The issue has been fixed by consistently setting GIC_PRIO_PSR_I_SET during exception entry to ensure kernel code sees a consistent environment. The fix includes updating local_daif_inherit() to handle PMR properly and removing special cases in the entry code. Users should upgrade to Linux kernel versions 5.10.38, 5.11.22, or 5.12.5 or later (Kernel Patch).