CVE-2025-21925 is a vulnerability identified in the Linux kernel, specifically related to the handling of socket buffers (sk_buff or skb) within the Logical Link Control (LLC) networking code and the e1000 Ethernet driver. The root cause involves improper use of the skb_get() function before calling dev_queue_xmit(), which is responsible for transmitting packets. The vulnerability arises because skb_get() increments the reference count of a shared skb, which can lead to a kernel crash when the e1000 driver calls eth_skb_pad() on a shared skb. This is problematic because eth_skb_pad() expects an unshared skb to safely pad the Ethernet frame, but the presence of shared skb buffers causes memory corruption or invalid operations, resulting in a kernel BUG and an invalid opcode exception. The issue was discovered by syzbot, an automated kernel fuzzing tool, which was able to crash hosts by exploiting this flaw using LLC and devices that do not support the IFF_TX_SKB_SHARING flag. The fix involves replacing skb_get() with skb_clone() in the net/llc/llc_s_ac.c source file, ensuring that a new skb copy is made rather than incrementing the reference count on a shared buffer. This prevents the kernel crash by avoiding modification of shared buffers. The vulnerability affects Linux kernel versions around 6.14.0-rc4 and potentially others using the e1000 driver and LLC networking code. There is also a note that the e1000 driver may have an unrelated issue with pktgen due to not clearing the IFF_TX_SKB_SHARING flag, but this is orthogonal to the vulnerability. No known exploits are reported in the wild yet, and no CVSS score has been assigned. The vulnerability can cause denial of service via kernel crashes, impacting system stability and availability.

For European organizations, this vulnerability poses a risk primarily to systems running vulnerable Linux kernel versions with the e1000 network driver enabled, which is common in many enterprise and cloud environments. The impact is mainly denial of service due to kernel crashes triggered by crafted network packets exploiting the skb reference counting flaw. This can disrupt critical services, especially in data centers, cloud providers, and network infrastructure relying on Linux servers. Systems used in telecommunications, financial services, and government sectors could face operational outages if exploited. Although no remote code execution or privilege escalation is indicated, the ability to crash the kernel remotely (via network packets) can be leveraged in targeted denial of service attacks. Given the widespread use of Linux in European IT infrastructure, including public sector and industrial control systems, the vulnerability could affect availability of essential services. The lack of known exploits reduces immediate risk, but the presence of an automated fuzzing tool discovery suggests potential for future exploitation. Organizations with high network traffic and exposure to untrusted networks are at greater risk.

European organizations should prioritize updating their Linux kernels to versions where this vulnerability is patched, specifically those that have replaced skb_get() with skb_clone() in the LLC networking code. Kernel updates should be applied promptly, especially on systems using the e1000 Ethernet driver. Network administrators should audit their environments for the presence of vulnerable kernel versions and drivers. As an interim mitigation, organizations can consider disabling LLC networking features if not required, or restricting network access to trusted sources to reduce exposure to crafted packets. Monitoring kernel logs for BUG or OOPS messages related to sk_buff or e1000 can help detect exploitation attempts. Additionally, organizations should review their use of pktgen and ensure network drivers are properly configured to avoid related issues. Implementing network segmentation and intrusion detection systems to detect anomalous LLC traffic patterns can further reduce risk. Finally, coordinating with Linux distribution vendors for timely security updates and advisories is recommended.