Shiftfs, an out-of-tree stacking file system included in Ubuntu Linux kernels, did not properly handle faults occurring during copy_from_user() correctly. These could lead to either a double-free situation or memory not being freed at all. An attacker could use this to cause a denial of service (kernel memory exhaustion) or gain privileges via executing arbitrary code. AKA ZDI-CAN-13562.
https://www.zerodayinitiative.com/advisories/ZDI-21-422/
https://www.openwall.com/lists/oss-security/2021/04/16/2
https://ubuntu.com/security/notices/USN-4917-1
http://packetstormsecurity.com/files/162614/Kernel-Live-Patch-Security-Notice-LSN-0077-1.html