CVE-2021-47254: 
Linux Kernel vulnerability analysis and mitigation

CVE-2021-47254 is a use-after-free vulnerability in the Linux kernel's GFS2 (Global File System 2) component, specifically in the gfs2_glock_shrink_scan function. The vulnerability was discovered and reported by syzbot, and was publicly disclosed on May 21, 2024. This vulnerability affects multiple versions of the Linux kernel up to versions 4.4.274, 4.9.274, 4.14.238, 4.19.196, 5.4.127, 5.10.45, and 5.12.12 (NVD).

The vulnerability occurs due to a race condition in the handling of the GLF_LRU flag under lru_lock. In the gfs2_glock_remove_from_lru() function, the GLF_LRU flag is checked under lru_lock to remove the glock from the lru list in __gfs2_glock_put(). However, on the shrink scan path, the same flag is cleared under lru_lock, but due to cond_resched_lock(&lru_lock) in gfs2_dispose_glock_lru(), progress on the put side can be made without deleting the glock from the lru list. The vulnerability has been assigned a CVSS v3.1 Base Score of 7.8 HIGH (AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H) (NVD).

This use-after-free vulnerability could potentially lead to privilege escalation, system crashes, or arbitrary code execution in the context of the kernel. The high CVSS score indicates that successful exploitation could result in a complete compromise of system confidentiality, integrity, and availability (NVD).

The vulnerability requires local access with low privileges to exploit. No known exploits have been publicly reported at the time of disclosure (NVD).

The vulnerability has been fixed by modifying the code to keep GLF_LRU across the race window opened by cond_resched_lock(&lru_lock) and clearing GLF_LRU after list_del under lru_lock. Users should update to patched kernel versions that include the fix (Kernel Patch).