I have traced this bug to a missing memory barrier in the powerpc IPI handling code. io_uring uses task_work_add() to schedule I/O worker creation, which in turn issues an IPI, and when precise timing conditions are met the inconsistent state between the two CPU cores can lead to corruption of userspace data in RAM.
I have sent a patch upstream, and created a merge request for Debian here: https://salsa.debian.org/kernel-team/linux/-/merge_requests/907
