CVE-2025-21664 is a vulnerability identified in the Linux kernel's device mapper thin provisioning (dm-thin) subsystem. The issue arises from improper handling of Read-Copy-Update (RCU) safe list operations in the get_first_thin() function. Specifically, the function originally used a sequence of list_empty() followed by list_first(), each performing independent READ_ONCE() operations on the list head. This can cause a race condition where list_empty() sees a non-empty list, but list_first() subsequently observes a different state due to concurrent modifications, leading to dereferencing invalid pointers. In practice, this flaw caused a production system crash due to a general protection fault in the process_deferred_bios path. The crash was triggered when get_first_thin() returned a pointer that was actually the list head itself (an empty list referencing itself), resulting in invalid memory access. Additional kernel warnings were observed, including refcount saturation and undefined behavior sanitizer (UBSAN) errors related to out-of-bounds CPU ID access in queued spinlocks. The root cause was a timing window where another thread removed the last entry from the active_thins list during synchronization, causing the race. The fix replaces the vulnerable code with list_first_or_null_rcu(), which performs a single READ_ONCE() operation and returns NULL if the list is empty, eliminating the race condition. Testing against the devicemapper thin provisioning test suite showed no regressions. This vulnerability affects Linux kernel versions identified by the commit hash b10ebd34cccae1b431caf1be54919aede2be7cbe and likely related versions. No known exploits are currently reported in the wild.

For European organizations, the impact of CVE-2025-21664 could be significant in environments relying on Linux servers using device mapper thin provisioning, especially in data centers, cloud infrastructure, and enterprise storage solutions. The vulnerability can cause kernel crashes leading to denial of service (DoS) conditions, potentially disrupting critical services and applications. Systems handling large-scale storage virtualization or thin provisioning are at higher risk. While the vulnerability does not directly enable privilege escalation or remote code execution, the resulting instability and crashes can impact availability and reliability of services. Organizations with high availability requirements, such as financial institutions, healthcare providers, and public sector entities, could face operational disruptions. Additionally, kernel crashes may lead to data corruption or loss if not properly managed. Since Linux is widely deployed across European IT infrastructure, especially in server and cloud environments, the scope of affected systems is broad. The absence of known exploits reduces immediate risk, but the complexity of the flaw and its subtle race condition nature mean that targeted attackers or accidental triggers could cause outages.

European organizations should prioritize updating Linux kernels to versions that include the patch fixing CVE-2025-21664. Specifically, ensure that the get_first_thin() function uses list_first_or_null_rcu() to prevent the race condition. System administrators should monitor kernel updates from trusted Linux distributions and apply them promptly. In environments where immediate patching is not feasible, consider implementing kernel crash monitoring and automated recovery mechanisms to minimize downtime. Additionally, review and test device mapper thin provisioning configurations to detect unusual behavior or instability. Employing kernel hardening techniques and enabling kernel debugging features can help identify early signs of exploitation or crashes. Organizations should also maintain robust backup and disaster recovery plans to mitigate potential data loss from unexpected kernel faults. Finally, coordinate with Linux distribution vendors and security teams to stay informed about further developments or related vulnerabilities.