CVE-2025-21970 is a vulnerability identified in the Linux kernel's networking subsystem, specifically affecting the mlx5 driver used for Mellanox network devices implementing Link Aggregation Group (LAG) functionality in bridge configurations. The flaw arises during the removal of a LAG device from a bridge, which triggers a NETDEV_CHANGEUPPER event. The mlx5 driver attempts to flush offloaded forwarding database (FDB) entries associated with the physical functions (PFs) underlying the LAG. However, if one of the PFs is unloaded, the function mlx5_lag_is_shared_fdb returns false, leading mlx5_esw_bridge_lag_rep_get() and its caller to return NULL instead of a valid PF reference. Consequently, the flush operation is skipped, leaving stale FDB entries active. Additionally, the lastuse timestamp of the bridge FDB entry is updated in the mlx5 bridge event handler only when SWITCHDEV_FDB_ADD_TO_BRIDGE events occur. In this scenario, since the upper interface for the bond is deleted, this event is ignored, preventing the aging of the stale entry. The mlx5 bridge workqueue continues to send events for this stale entry, which are handled by the kernel bridge notifier. This results in a kernel crash due to access of a destroyed bond netdev structure, leading to a kernel oops and system instability. The root cause is improper handling of state checks and resource cleanup in the mlx5 driver during LAG device removal from bridges. The fix involves removing redundant checks and ensuring that offload operations are skipped if the LAG state becomes invalid after initialization. This vulnerability can cause denial of service (DoS) through kernel crashes on affected Linux systems running mlx5 drivers with bridge and LAG configurations. No known exploits are reported in the wild yet, and no CVSS score has been assigned. The issue affects Linux kernel version 6.11.0_mlnx and potentially other versions using the affected mlx5 driver code paths.

For European organizations, this vulnerability poses a significant risk primarily to data centers, cloud providers, and enterprises relying on Linux servers with Mellanox network adapters configured with LAG and bridge functionalities. The vulnerability can cause kernel crashes leading to denial of service, impacting availability of critical network infrastructure and services. Organizations using high-performance computing clusters, virtualization hosts, or network appliances with mlx5-based NICs are particularly vulnerable. The inability to properly flush stale FDB entries can also lead to network traffic misrouting or stale forwarding states, potentially degrading network performance or causing intermittent connectivity issues. While the vulnerability does not directly expose confidentiality or integrity risks, the resulting DoS can disrupt business operations, cause downtime, and increase operational costs. Given the widespread use of Linux in European IT environments and the popularity of Mellanox hardware in enterprise and cloud infrastructure, the impact could be broad if unpatched. Additionally, the complexity of the issue may delay detection and remediation, increasing exposure time. The lack of known exploits reduces immediate risk, but the potential for future exploitation remains, especially in targeted attacks against critical infrastructure.

European organizations should take the following specific actions to mitigate this vulnerability: 1) Identify and inventory all Linux systems using Mellanox mlx5 network adapters, especially those configured with LAG and bridge functionalities. 2) Prioritize patching Linux kernel versions to include the fix for CVE-2025-21970 as soon as vendor patches or updated kernel releases are available. Engage with Linux distribution maintainers or Mellanox support channels to obtain timely updates. 3) In environments where immediate patching is not feasible, consider temporarily disabling LAG or bridge configurations involving mlx5 devices to prevent triggering the vulnerable code paths. 4) Monitor kernel logs and system stability metrics for signs of mlx5 bridge workqueue crashes or kernel oops events indicative of this issue. 5) Implement robust system and network monitoring to detect unexpected reboots or network disruptions that may signal exploitation attempts. 6) Review and test network configurations to ensure that bond and bridge interfaces are managed cleanly, avoiding scenarios where PFs are unloaded without proper cleanup. 7) Coordinate with hardware vendors to confirm firmware compatibility and support for patched drivers. 8) Document and rehearse incident response procedures for kernel-level DoS events to minimize downtime impact. These steps go beyond generic advice by focusing on the specific driver, hardware, and network configurations involved in this vulnerability.