CVE-2021-46968: 
Linux Kernel vulnerability analysis and mitigation

CVE-2021-46968 affects the Linux kernel and involves a memory leak vulnerability in the s390/zcrypt component. The issue was discovered when tests with kvm and a kmemdebug kernel revealed that during hot unplug operations, the zcard and zqueue structs for unplugged cards or queues were not properly freed due to a mismatch with get/put operations for the embedded kref counter (Kernel Patch).

The vulnerability stems from a reference counting issue where the kref counter, which starts with a value of 1 during initialization, fails to properly drop to zero when unregistering the card or queue. This prevents the release and freeing of the object, resulting in a memory leak. The issue affects Linux kernel versions from 5.10 up to (excluding) 5.10.36, 5.11 up to (excluding) 5.11.20, and 5.12 up to (excluding) 5.12.3. The vulnerability has been assigned a CVSS v3.1 base score of 5.5 MEDIUM (AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H) (NVD).

The vulnerability results in a memory leak condition when hot-unplugging crypto cards and queues on s390 systems. While there are no direct confidentiality or integrity impacts, the memory leak can lead to resource exhaustion over time, potentially affecting system availability (NVD).

The vulnerability requires local access with low privileges to exploit. The attack complexity is low, and no user interaction is required. However, the scope is unchanged, meaning the impact is limited to the vulnerable component (NVD).

The vulnerability has been fixed by adjusting the handling of kref counters to ensure proper memory cleanup during hot-unplug operations. The fix involves adding proper zcrypt_card_put(zc) and zcrypt_queue_put(zq) calls in the unregister functions. Users should upgrade to Linux kernel versions 5.10.36, 5.11.20, 5.12.3 or later to receive the fix (Kernel Patch).