All the uses of vmsplice etc are a bit tricky, and that points to the need for a better interface. But given you're using splice, why not do the crypto in user space? A belief that it is better to be fast and buggy than safe and slower?
Still a risk that some admin-enabled method (like enabling an IPsec VPN) provides a path to it, but would reduce the potential for crafting weird inputs.
That's really orthogonal (and you can already do io_uring with AF_ALG, at the end of the day AF_ALG is just recvmsg() and sendmsg(), which work just fine in io_uring...)
