CVE-2025-21862 is a vulnerability identified in the Linux kernel specifically related to the drop_monitor kernel module. The issue arises due to an incorrect initialization order of resources within the drop_monitor component. When drop_monitor is built as a kernel module, there exists a race condition during module loading where the Syzkaller fuzzing tool can trigger a netlink NET_DM_CMD_START message. This message invokes the net_dm_monitor_start() function, which attempts to use a spinlock that has not yet been initialized. The improper initialization leads to a 'spinlock bad magic' error, indicating that the spinlock's internal state is invalid (magic value zero, no owner). This can cause kernel instability, crashes (kernel panic), or undefined behavior due to improper synchronization primitives being used. The root cause is that resource initialization occurs after the registration of a generic netlink family, allowing the race condition to manifest. The fix involves reordering the initialization sequence to ensure that all resources, including the spinlock, are fully initialized before the netlink family registration. This vulnerability was discovered by InfoTeCS on behalf of the Linux Verification Center using the Syzkaller fuzzing tool. It affects Linux kernel versions including commit 9a8afc8d3962f3ed26fd6b56db34133860ed1e72 and potentially others in the 5.10.x series or similar. No known exploits are currently reported in the wild, and no CVSS score has been assigned yet.

For European organizations, this vulnerability poses a risk primarily to systems running affected Linux kernel versions with the drop_monitor module enabled or loaded as a kernel module. The impact includes potential denial of service (DoS) through kernel crashes or instability, which can disrupt critical services, especially in environments relying on Linux servers, virtual machines, or container hosts. Since the vulnerability involves kernel-level synchronization primitives, exploitation could lead to system-wide instability affecting confidentiality, integrity, and availability indirectly by causing unexpected reboots or service interruptions. Although no direct privilege escalation or remote code execution is indicated, the ability to cause kernel panics can be leveraged by attackers to disrupt operations or as part of a multi-stage attack. European sectors with high reliance on Linux infrastructure, such as telecommunications, finance, cloud service providers, and public sector IT, could experience operational disruptions if unpatched. The vulnerability is more relevant in environments where kernel modules are dynamically loaded and where fuzzing or malformed netlink messages could be sent, including multi-tenant cloud environments and virtualized platforms.

1. Immediate patching: Apply the official Linux kernel updates that reorder the initialization sequence in the drop_monitor module to ensure spinlocks are properly initialized before use. 2. Disable drop_monitor module if not required: Organizations should audit their kernel modules and disable or blacklist drop_monitor if it is not essential to their operations, reducing the attack surface. 3. Harden netlink message handling: Implement network-level controls to restrict or monitor netlink message traffic, especially in multi-tenant or virtualized environments, to prevent unauthorized triggering of net_dm_monitor_start(). 4. Kernel module loading policies: Enforce strict kernel module loading policies and integrity checks to prevent unauthorized or malicious module loading. 5. Monitoring and alerting: Deploy kernel crash monitoring and alerting mechanisms to detect early signs of exploitation attempts or instability related to spinlock misuse. 6. Use of fuzzing tools: Employ fuzz testing in controlled environments to proactively identify similar race conditions or initialization order bugs in custom or third-party kernel modules.