CVE-2025-21868 is a vulnerability identified in the Linux kernel networking subsystem, specifically related to the handling of network buffer fragments (SKBs) and the small head cache usage when configured with large MAX_SKB_FRAGS values. The vulnerability arises when the kernel is built with a configuration where MAX_SKB_FRAGS is set to 45, causing the SKB_WITH_OVERHEAD(1024) size to be smaller than GRO_MAX_HEAD. This mismatch leads to improper handling of network packet fragments in the kernel's network stack, particularly in the functions netif_napi_add_weight_locked and napi_get_frags. The issue manifests as a kernel crash (splat) due to an invalid memory access or improper cache usage, triggered during network initialization routines such as gro_cells_init, xfrm_input_init, and ip_rt_init. The root cause is the small head cache not being sized correctly to accommodate both TCP and Generic Receive Offload (GRO) allocations, resulting in the fallback to the page fragment allocator that triggers the crash. The vulnerability is independent of a related revert of the single page frag cache but requires updates to napi_alloc_skb() and __netdev_alloc_skb() to ensure kmalloc() is used for allocations fitting the small head cache. This flaw affects Linux kernel versions built with the specified MAX_SKB_FRAGS configuration and can cause denial of service through kernel panics during network 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 Linux kernels compiled with the affected MAX_SKB_FRAGS configuration, which may include custom or specialized Linux distributions used in network infrastructure, servers, and embedded devices. The impact is mainly a denial of service (DoS) condition caused by kernel crashes during network packet processing, which can disrupt critical network services, degrade availability, and potentially lead to system reboots or downtime. Organizations relying on Linux-based networking equipment, cloud infrastructure, or data centers could experience service interruptions. Although the vulnerability does not directly lead to privilege escalation or data leakage, the resulting instability can affect business continuity and operational reliability. The absence of known exploits reduces immediate risk, but the complexity of the issue and its presence in core kernel networking code necessitate prompt attention to prevent exploitation or accidental triggering in production environments.

Mitigation requires applying kernel patches that address the small head cache sizing issue and update the skb allocation functions (napi_alloc_skb() and __netdev_alloc_skb()) to use kmalloc() for appropriate allocations. Organizations should: 1) Identify Linux systems running kernels with MAX_SKB_FRAGS=45 or similar configurations; 2) Monitor vendor advisories and apply official Linux kernel updates or backported patches that fix CVE-2025-21868; 3) For custom kernel builds, adjust the MAX_SKB_FRAGS setting and ensure the small head cache is sized to accommodate TCP and GRO allocations correctly; 4) Test kernel updates in staging environments to verify stability; 5) Implement network segmentation and redundancy to minimize impact of potential DoS conditions; 6) Monitor system logs for kernel warnings or crashes related to netif_napi_add_weight_locked or napi_get_frags functions; 7) Engage with Linux distribution maintainers or vendors for guidance on patch availability and deployment timelines.