There is a logic error in io_uring's implementation which can be used to trigger a use-after-free vulnerability leading to privilege escalation. In the io_prep_async_work function the assumption that the last io_grab_identity call cannot return false is not true, and in this case the function will use the init_cred or the previous linked requests identity to do operations instead of using the current identity. This can lead to reference counting issues causing use-after-free.

Upstream fix:
https://github.com/torvalds/linux/commit/1e6fa5216a0e59ef02e8b6b40d553238a3b81d49