CVE-2021-47261 is a vulnerability identified in the Linux kernel's InfiniBand (IB) mlx5 driver, specifically related to the initialization of Completion Queue (CQ) fragments buffers. The issue arises in the function init_cq_frag_buf(), which is responsible for initializing either the current CQ fragments buffer (cq->buf) or a temporary buffer (cq->resize_buf) used during CQ resize operations. The vulnerability stems from a code change that replaced direct buffer access with calls to get_cqe(), a function that always returns CQEs from the current buffer (cq->buf) regardless of whether the operation involves resizing and thus should use the temporary buffer. This incorrect buffer reference leads to out-of-bounds access when enlarging the CQ, as the code attempts to access elements beyond the size of the current buffer, resulting in a kernel panic (system crash). The stack trace provided shows the panic occurring within mlx5_ib_resize_cq and related IB core functions. The fix involves replacing get_cqe() calls with mlx5_frag_buf_get_wqe(), which correctly takes the source buffer as a parameter, ensuring the right buffer is accessed during initialization and resizing. This vulnerability affects Linux kernel versions identified by the commit hash 388ca8be00370db132464e27f745b8a0add19fcb and was published on May 21, 2024. There are no known exploits in the wild as of the publication date, and no CVSS score has been assigned yet.

The primary impact of CVE-2021-47261 is a denial of service (DoS) condition caused by a kernel panic when the vulnerable mlx5 driver attempts to resize Completion Queues incorrectly. For European organizations, especially those operating data centers, high-performance computing clusters, or enterprise environments relying on InfiniBand networking for low-latency, high-throughput communication, this vulnerability could lead to unexpected system crashes, disrupting critical services and workloads. The kernel panic could affect servers running Linux kernels with the vulnerable mlx5 driver, potentially causing downtime and impacting business continuity. While the vulnerability does not appear to allow privilege escalation or remote code execution, the availability impact on systems using InfiniBand hardware with the mlx5 driver is significant. Organizations in sectors such as finance, research, telecommunications, and cloud service providers in Europe that use InfiniBand for their internal networks or HPC clusters are particularly at risk. The absence of known exploits reduces immediate risk, but unpatched systems remain vulnerable to accidental or malicious triggering of the kernel panic, which could be leveraged in targeted attacks or cause operational instability.

European organizations should prioritize updating their Linux kernel versions to include the patch that replaces get_cqe() with mlx5_frag_buf_get_wqe() in the mlx5 driver. Since the vulnerability is in the kernel driver code, applying the latest stable kernel updates from trusted Linux distributions (e.g., Debian, Ubuntu, Red Hat, SUSE) that incorporate this fix is the most effective mitigation. For environments where immediate kernel upgrades are challenging, organizations should consider temporarily disabling or avoiding the use of InfiniBand mlx5 devices or CQ resizing operations if feasible. Monitoring kernel logs for signs of kernel panics related to mlx5_ib_resize_cq can help detect attempts to trigger the vulnerability. Additionally, organizations should review and test their high-performance networking configurations to ensure stability post-patch. Network segmentation and strict access controls to systems with InfiniBand hardware can reduce the risk of exploitation attempts. Finally, maintaining robust backup and recovery procedures will help mitigate the impact of any unexpected downtime caused by this vulnerability.