CVE-2021-46990 is a vulnerability identified in the Linux kernel specifically affecting the powerpc/64s architecture. The issue arises from the kernel's handling of the entry flush mitigation, which is a security feature that can be toggled at runtime via a debugfs interface (entry_flush). This mitigation involves the kernel patching itself dynamically to enable or disable certain protections. The vulnerability occurs because the kernel may perform this patching while other CPUs are still active, leading to a race condition. This unsafe patching can cause kernel crashes or corruption of the link register (LR), which is critical for returning from kernel mode to user mode. The corrupted LR points into the kernel space, resulting in a segmentation fault and potential system instability or denial of service. The root cause is that the kernel does not stop all CPUs during the patching process, allowing some CPUs to execute partially patched code. The fix implemented involves performing the patching under a 'stop machine' context, which halts all other CPUs during the patching operation, ensuring no CPU executes inconsistent code. This approach prevents the race condition and stabilizes the kernel behavior during mitigation toggling. The vulnerability affects multiple Linux kernel versions identified by specific commit hashes, indicating it is present in various recent kernel builds prior to the fix. There are no known exploits in the wild, and no CVSS score has been assigned yet. The vulnerability primarily impacts systems running Linux on powerpc/64s architectures that utilize the entry flush mitigation feature.

For European organizations, the impact of CVE-2021-46990 is primarily related to system stability and availability on affected Linux systems using the powerpc/64s architecture. Organizations relying on Linux servers or embedded systems with this architecture could experience unexpected crashes or denial of service if the entry flush mitigation is toggled at runtime. This could disrupt critical services, especially in sectors like telecommunications, research institutions, or industries using specialized hardware based on powerpc/64s. Although the vulnerability does not directly lead to privilege escalation or data breach, the resulting system crashes could cause operational downtime, data loss, or interruption of business processes. Since the vulnerability requires kernel-level interaction and affects low-level CPU operations, it is unlikely to be exploited remotely without prior access. However, insider threats or compromised administrators could trigger the vulnerability by toggling the mitigation improperly. The absence of known exploits reduces immediate risk, but unpatched systems remain vulnerable to stability issues. European organizations with infrastructure running affected kernel versions should prioritize patching to maintain system reliability and avoid potential service disruptions.

1. Apply the official Linux kernel patches that implement the 'stop machine' mechanism during entry flush mitigation toggling. Ensure all affected systems are updated to kernel versions containing the fix. 2. Avoid toggling the entry flush mitigation at runtime unless absolutely necessary, especially on production systems. 3. Restrict access to debugfs interfaces, particularly the entry_flush file, to trusted administrators only, minimizing the risk of accidental or malicious toggling. 4. Monitor system logs for signs of segmentation faults or crashes related to the entry flush mitigation and investigate promptly. 5. For organizations using custom or embedded Linux distributions, coordinate with vendors to ensure timely integration of the patch. 6. Implement robust change management and kernel update procedures to ensure all Linux systems are regularly updated and tested for stability. 7. Consider isolating critical systems running powerpc/64s architectures to limit exposure and reduce the risk of exploitation or accidental triggering of the vulnerability.