CVE-2021-47044 is a high-severity vulnerability in the Linux kernel's scheduler subsystem, specifically within the fair scheduler's load_balance() function. The issue arises from an unchecked shift operation that can lead to a shift-out-of-bounds condition, which is undefined behavior in C. The vulnerability was identified by Syzbot, which observed unusually high values for the variable sd->nr_balance_failed, a counter that increments on failed load balancing attempts. Normally, when this counter reaches a threshold (sd->cache_nice_tries + 3), an active balancing attempt is triggered to reset or adjust the counter. However, if the task selected for balancing is not allowed to run on the target CPU (env->dst_cpu), the counter increments without the expected reset or adjustment, allowing it to grow indefinitely. This unchecked growth can cause a left shift operation to exceed the bit-width of the operand, leading to undefined behavior, potential memory corruption, or kernel instability. The root cause is that the shift exponent was not capped properly, violating safe shift operation constraints. The fix involves capping the shift exponent to BITS_PER_TYPE(typeof(lefthand)) - 1 to prevent out-of-bounds shifts. This vulnerability is classified under CWE-125 (Out-of-bounds Read) and has a CVSS 3.1 score of 7.7, indicating high severity. Exploitation requires local access (AV:L), no privileges (PR:N), and no user interaction (UI:N), but it impacts confidentiality and availability significantly. No known exploits are reported in the wild as of the publication date.

For European organizations, this vulnerability poses a significant risk primarily to servers and systems running vulnerable Linux kernel versions, especially those using the fair scheduler in multi-core environments. The flaw can lead to kernel crashes or unpredictable behavior, causing denial of service (DoS) conditions that disrupt critical services. Confidentiality impact is high because kernel memory corruption could potentially be leveraged to leak sensitive information, although no direct integrity impact is noted. Systems in data centers, cloud infrastructure, and enterprise environments relying on Linux for workload scheduling are at risk. The vulnerability's local attack vector means that attackers need some form of access, such as through compromised user accounts or malicious insiders, to exploit it. Given the widespread use of Linux in European government, financial, telecommunications, and industrial sectors, the potential for service disruption and data exposure is considerable. The lack of known exploits reduces immediate risk but does not eliminate the threat, especially as attackers may develop exploits over time.

European organizations should prioritize patching Linux kernels to versions where this vulnerability is fixed, ensuring the load_balance() function no longer performs unsafe shift operations. Kernel updates should be tested and deployed promptly in production environments. For systems where immediate patching is not feasible, organizations should restrict local access to trusted users only, implement strict access controls, and monitor for anomalous kernel behavior or crashes indicative of exploitation attempts. Employing kernel hardening techniques such as Kernel Address Space Layout Randomization (KASLR) and enabling security modules like SELinux or AppArmor can reduce exploitation likelihood. Additionally, organizations should audit and limit the use of multi-core scheduling policies that might trigger the vulnerable code path, if possible. Continuous monitoring for unusual load balancing failures or kernel logs referencing sd->nr_balance_failed anomalies can provide early warning signs. Finally, coordinate with Linux distribution vendors for timely security advisories and patches.