CVE-2021-47337 is a vulnerability identified in the Linux kernel's SCSI (Small Computer System Interface) core subsystem. The flaw arises from improper error handling in the function scsi_host_alloc(), specifically related to the management of the error handler kernel thread (ehandler) associated with SCSI hosts. When the kernel attempts to allocate a SCSI host, it spawns an error handler kthread to manage error conditions. However, if this kthread fails to spawn (e.g., due to memory allocation failure), the pointer shost->ehandler is set to an error pointer ERR_PTR(-ENOMEM) rather than NULL. Later, during cleanup in scsi_host_dev_release(), the code checks if shost->ehandler is non-NULL and calls kthread_stop() unconditionally. Since shost->ehandler is an invalid error pointer and not a valid thread pointer, this leads to a kernel NULL pointer dereference and a consequent kernel panic or crash. The vulnerability is triggered when the error handler thread fails to spawn, which is an unlikely but possible scenario. The kernel oops logs show a NULL pointer dereference at a low memory address during kthread_stop(), confirming the dereference of an invalid pointer. The root cause is a missing nullification of shost->ehandler when the kthread creation fails. The fix involves explicitly setting shost->ehandler to NULL if the error handler thread fails to spawn, preventing the invalid dereference. This vulnerability affects multiple versions of the Linux kernel as indicated by the affected commit hashes. It is a stability and availability issue that can cause denial of service (DoS) by crashing the kernel. There is no indication of privilege escalation or direct code execution, but the kernel crash can disrupt system operations. No known exploits in the wild have been reported to date. The vulnerability does not have a CVSS score assigned yet.

For European organizations, the primary impact of CVE-2021-47337 is a potential denial of service condition on Linux systems utilizing the affected kernel versions, particularly those using SCSI devices or subsystems such as IBM Virtual SCSI (ibmvscsi). Organizations relying on Linux servers for critical infrastructure, cloud services, or enterprise applications could experience unexpected kernel panics leading to system downtime, data unavailability, and operational disruption. This is especially critical for data centers, hosting providers, and industries with high availability requirements such as finance, healthcare, and manufacturing. Although exploitation requires the error handler thread to fail spawning, which is uncommon, the vulnerability could be triggered by resource exhaustion or malicious attempts to destabilize the system. The vulnerability does not appear to allow privilege escalation or arbitrary code execution, so confidentiality and integrity impacts are limited. However, availability impacts could be significant if exploited or triggered inadvertently. Systems running older or unpatched Linux kernels are at risk until updated. Given the widespread use of Linux in European IT environments, including public sector and private enterprises, the risk of disruption is non-negligible. The lack of known exploits reduces immediate threat but patching is recommended to prevent potential future abuse or accidental crashes.

1. Apply the official Linux kernel patches that fix CVE-2021-47337 as soon as possible. This involves updating to a kernel version that includes the fix where shost->ehandler is properly nullified upon kthread spawn failure. 2. For organizations using custom or long-term support kernels, backport the patch to ensure the fix is included. 3. Monitor kernel logs for error handler thread spawn failures or related kernel oops messages to detect attempts to trigger this vulnerability. 4. Implement resource management and limits to prevent conditions that could cause kthread spawn failures, such as memory exhaustion or excessive kernel thread creation. 5. Use kernel hardening and security modules that can help detect and mitigate kernel crashes or abnormal behavior. 6. Test kernel updates in staging environments before deployment to critical systems to avoid unexpected regressions. 7. Maintain an inventory of Linux kernel versions in use across the organization to prioritize patching efforts. 8. Consider isolating critical workloads or using containerization to limit the impact of kernel crashes on overall infrastructure. 9. Engage with Linux distribution vendors for timely security updates and advisories related to this vulnerability.