Does this mean to successfully exploit a program I need to write to an area in memory that the program will later turn the page in memory to "Execute"?
You can try return-oriented programming, but OpenBSD makes that hard: the stack is never executable, everything is position independent, objects get shuffled around inside libraries and executables¹, maybe more.
¹ At least this was proposed, and I think it landed in one of the newer releases, but I can't seem to find it for sure.
Found the slide that I remembered: https://twitter.com/justincormack/status/577005049374601217
Do you know if any of the rest of that is in-progress or complete?
* binutils 2.17 (last GPLv2) - complete.
* all archs transitioned to pie - complete as of 5.7.
* /sbin/init static pie - complete.
* ROP gadgets are being hunted down systematically, many nop sleds changed to int3/illegal instr.
* BROP - many base daemons now fork + exec, not all.
* SROP - mitigation committed - complete?
* shuffle - libc objects get shuffled at boot by rc(8), stack object order shuffling as a gcc extension.
..
Progress continues.