CVE-2021-46984 is a vulnerability in the Linux kernel's block I/O scheduler subsystem, specifically affecting the Kyber I/O scheduler. The issue arises from an out-of-bounds memory access caused by a race condition related to CPU preemption during context retrieval. The function __blk_mq_sched_bio_merge() obtains the context (ctx) and hardware context (hctx) for the current CPU and passes hctx to the bio_merge() callback. Kyber's bio_merge implementation (kyber_bio_merge()) then retrieves the ctx again for the current CPU to map it to the corresponding Kyber context within the passed hctx. However, if the thread is preempted between these two calls, the second ctx may no longer correspond to the original hctx, potentially leading to reading invalid or garbage data. This mismatch can cause an out-of-bounds array access, as evidenced by the UBSAN (Undefined Behavior Sanitizer) error indicating an index far exceeding the array bounds. The root cause is that Kyber uses hctx to map contexts, whereas other schedulers like BFQ and mq-deadline use the request_queue directly, avoiding this mismatch. The fix involves passing the request_queue to bio_merge() instead of hctx, allowing Kyber to map queues internally and prevent the race condition. This vulnerability is a kernel-level memory safety issue that can lead to undefined behavior, including potential kernel crashes or data corruption. It affects Linux kernel versions identified by the given commit hashes prior to the patch and does not require user interaction or authentication to be triggered, as it occurs during normal block I/O operations. No known exploits are reported in the wild as of the publication date.

For European organizations, this vulnerability poses a risk primarily to systems running affected Linux kernel versions with the Kyber I/O scheduler enabled. Since Kyber is the default I/O scheduler in many modern Linux distributions, including those used in enterprise servers, cloud infrastructure, and embedded systems, the scope is broad. Exploitation could lead to kernel crashes (denial of service), data corruption, or potentially privilege escalation if an attacker can manipulate block I/O operations. This could disrupt critical services, especially in data centers, cloud providers, and industries relying on Linux-based storage systems. The impact on confidentiality is limited unless combined with other vulnerabilities, but integrity and availability could be significantly affected. Given the kernel-level nature, recovery may require system reboots and patching, impacting operational continuity. Organizations with high I/O workloads or those using Kyber in storage-heavy environments (e.g., databases, virtualized infrastructure) are particularly at risk. The absence of known exploits reduces immediate threat but does not eliminate the risk, especially from targeted attackers or advanced persistent threats.

European organizations should: 1) Identify and inventory Linux systems running affected kernel versions with Kyber enabled. 2) Apply the official Linux kernel patches that address CVE-2021-46984 as soon as they become available from trusted sources or distribution vendors. 3) If immediate patching is not possible, consider temporarily switching the I/O scheduler to a non-Kyber scheduler (e.g., BFQ or mq-deadline) to mitigate the risk of exploitation. 4) Monitor kernel logs and system behavior for signs of out-of-bounds errors, crashes, or unusual I/O failures that could indicate exploitation attempts. 5) Employ strict access controls and limit untrusted user access to systems performing block I/O operations to reduce attack surface. 6) Incorporate this vulnerability into vulnerability management and incident response plans to ensure timely detection and remediation. 7) Engage with Linux distribution vendors to receive timely updates and security advisories. These steps go beyond generic advice by focusing on scheduler configuration and proactive monitoring specific to this vulnerability.