CVE-2021-47221 is a vulnerability identified in the Linux kernel's SLUB memory allocator subsystem, specifically related to the redzoning feature used for debugging memory corruption issues. The SLUB allocator uses redzones—guard areas around allocated memory objects—to detect buffer overflows and memory corruption. The vulnerability arises because the redzoning checks incorrectly use the s->object_size field instead of s->inuse to determine the bounds for the freelist pointer placement. In cases where the cache object size is less than 24 bytes, the freelist pointer can overwrite the redzone area, corrupting it. This leads to false positives or missed detections of memory corruption when the "slub_debug=Z" or "slub_debug=ZF" debugging options are enabled. The issue was identified through kernel debug messages indicating right redzone overwrites and has been fixed by adjusting the offset calculations to stay within the object size boundaries. Importantly, the advisory notes that no known kernel caches currently use object sizes in the vulnerable range, suggesting limited practical exposure. There are no known exploits in the wild, and no CVSS score has been assigned yet. The vulnerability primarily affects Linux kernel versions prior to the patch and is relevant to systems using SLUB with redzoning enabled for debugging purposes.

For European organizations, the direct impact of CVE-2021-47221 is likely limited due to the nature of the vulnerability and its debugging-specific context. Since the flaw affects the SLUB allocator's redzoning feature, which is typically enabled only in development or debugging environments, production systems running standard Linux kernel configurations without these debug flags are unlikely to be affected. However, organizations that perform kernel debugging or develop kernel modules may encounter false positives or instability during memory corruption detection, potentially delaying vulnerability assessments or kernel development activities. In rare cases, if a custom kernel or specialized embedded system uses small object caches with sizes under 24 bytes and enables SLUB redzoning, there could be memory corruption risks leading to system crashes or unpredictable behavior. This could impact critical infrastructure or industrial control systems running Linux kernels with these configurations. Overall, the vulnerability does not present a direct attack vector for remote exploitation or privilege escalation but may affect kernel debugging reliability and system stability in niche scenarios.

European organizations should ensure that Linux kernel versions are updated to include the patch fixing CVE-2021-47221, especially in environments where kernel debugging with SLUB redzoning is used. Specifically, kernel developers and maintainers should verify that their build configurations do not enable "slub_debug=Z" or "slub_debug=ZF" on production systems unless necessary. For development and testing environments, apply the patch to avoid false positives and redzone corruption. Additionally, organizations using custom or embedded Linux kernels should audit their kernel cache configurations to confirm no caches exist with object sizes less than 24 bytes, or adjust configurations accordingly. Monitoring kernel debug logs for redzone overwrite messages can help detect residual issues. Since no known exploits exist, prioritizing this fix can be balanced against other critical patches, but it should be included in routine kernel maintenance and updates. Finally, maintain good kernel debugging practices and isolate debug-enabled systems from production environments to minimize risk.