CVE-2021-46990: 
Linux Kernel vulnerability analysis and mitigation

CVE-2021-46990 affects the Linux kernel's powerpc/64s architecture implementation. The vulnerability was discovered in the entry flush mitigation feature that can be enabled/disabled at runtime via a debugfs file (entry_flush). This issue affects multiple versions of the Linux kernel, including versions from 4.4.245 through 5.13-rc7 (NVD).

The vulnerability occurs when toggling the entry flush barrier, which causes the kernel to patch itself to enable/disable relevant mitigations. The issue arises when patching occurs while other CPUs are active, leading to unsafe execution conditions. For example, a crash scenario shows a segfault at address c000000000004c20 with corrupted LR (Link Register) pointing into the kernel, due to executing partially patched call to the fallback entry flush. The vulnerability has been assigned a CVSS v3.1 Base Score of 5.5 MEDIUM (Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H) (NVD).

The vulnerability can result in system crashes when the entry flush mitigation is toggled, potentially leading to denial of service conditions. The issue specifically affects the powerpc/64s architecture and can cause the system to return to userspace with a corrupted Link Register (Kernel Patch).

The vulnerability requires local access and low privileges to exploit. The attack complexity is low, but no user interaction is needed. The vulnerability can be triggered through the debugfs interface when toggling the entry flush barrier (NVD).

The issue has been fixed by implementing patching under stop machine conditions. The fix ensures that CPUs not doing the patching will spin in the core of the stop machine logic with interrupts hard disabled. This approach is sufficient as none of the patching affects the stop machine code or nearby areas (Kernel Patch).