The io_uring API is fairly narrow. It's basically the same interface as the kernel uses to talk to network cards and modern disks. Those are untrusted just like userspace is. You can look at the packet format here (section 4.1):

https://kernel.dk/io_uring.pdf

It's not hard to parse, and nowhere near the level of exposure eBPF creates.

If you don't trust the hardware MMU to allow the kernel to safely read buffers from userspace, then there's really no way to perform I/O in the first place. (write() already does this, for example).