CVE-2020-14356: Use After Free

A use-after-free flaw was found in the Linux kernel’s cgroupv2 subsystem when rebooting the system. This flaw allows a local user to crash the system or escalate their privileges. The highest threat from this vulnerability is to system availability.

It was found that the Linux kernel's use after free issue could happen during the usage of cgroupv2 (usually when reboot and more likely if virtual machine or docker being used). After recent commit 090e28b ("netpriocgroup: Fix unlimited memory leak of v2 cgroups") was merged, the problem could be reproduced much simpler starting with 5.7.x kernels. The problem introduced with 4.5 kernel, but there is no known way to trigger it, so it was not known until 5.7 kernel. It could be possible to reproduce the bug by the user without privileges and without reboot if user has an ability to create/close cgroupv2 or this user create a process which is attached to already existing cgroupv2 BPF (but still the user will have to wait till root manually closes cgroupv2 or system is being rebooted).

This commit fixes the problem: https://github.com/torvalds/linux/commit/94886c86e833dbc8995202b6c6aaff592b7abd24

— Red Hat