CVE-2021-46981 is a vulnerability identified in the Linux kernel's Network Block Device (nbd) subsystem. The flaw arises due to improper handling of NULL pointers during the disconnection process of /dev/nbdX devices. Specifically, when /dev/nbdX is opened, the configuration reference count (config_refs) is set to 1, but the pointers within the nbd_device structure remain NULL. If the device is then disconnected, the code attempts to reference a NULL recv_workq pointer, leading to a NULL pointer dereference. The protection mechanism using config_refs in the nbd_genl_disconnect function is ineffective in preventing this dereference. This results in a kernel oops and crash, as evidenced by the kernel logs showing a supervisor write access fault and a NULL pointer dereference at address 0x20. The root cause is the lack of a NULL check before accessing recv_workq in the nbd_disconnect_and_put function. The fix involves adding a conditional check to ensure recv_workq is not NULL before it is dereferenced. This vulnerability affects multiple versions of the Linux kernel as indicated by the affected commit hashes. Exploitation does not require user interaction beyond opening and disconnecting the nbd device, and it can cause a denial of service by crashing the kernel. There are no known exploits in the wild at the time of publication, and no CVSS score has been assigned yet. The vulnerability was published on February 28, 2024, and is relevant to Linux kernel versions that include the affected nbd code.

For European organizations, the primary impact of CVE-2021-46981 is the potential for denial of service (DoS) attacks on Linux systems utilizing the Network Block Device functionality. This could disrupt critical services, especially in environments relying on nbd for networked storage solutions, such as cloud providers, data centers, and enterprises using Linux-based storage virtualization. A kernel crash can lead to system downtime, loss of availability, and potential data corruption if the crash occurs during critical I/O operations. While this vulnerability does not directly lead to privilege escalation or remote code execution, the resulting instability can be exploited by attackers to cause service interruptions. Organizations with infrastructure running affected Linux kernel versions are at risk, particularly those with automated or remote management systems that might trigger nbd device operations. The lack of requirement for user interaction means that an attacker with local access or the ability to send commands to affected systems could trigger the vulnerability. Given the widespread use of Linux in European IT infrastructure, especially in cloud and server environments, the impact could be significant if unpatched systems are targeted.

To mitigate CVE-2021-46981, European organizations should: 1) Apply the official Linux kernel patches that add the necessary NULL pointer checks in the nbd_disconnect_and_put function as soon as they become available from their Linux distribution vendors. 2) If immediate patching is not possible, consider disabling the nbd kernel module or avoiding the use of /dev/nbdX devices to prevent exposure. 3) Implement strict access controls to limit which users and processes can open or disconnect nbd devices, reducing the risk of exploitation by unauthorized actors. 4) Monitor kernel logs for signs of NULL pointer dereferences or kernel oops related to nbd to detect potential exploitation attempts. 5) Employ kernel live patching solutions where supported to apply fixes without requiring full system reboots, minimizing downtime. 6) Incorporate this vulnerability into vulnerability management and incident response plans to ensure timely detection and remediation. 7) Educate system administrators about the risks associated with nbd usage and the importance of applying security updates promptly.