A workaround might be to make all setuid/setgid files non-world-readable because then they cannot be opened at all, and thus there is no setuid file to replace the contents of.

Eh, if you can pollute page caches this won’t safe you.

Think modifying shared libraries, ld preload, cron, I guess on some systems /etc/passwd even.

There are a lot of files readable that should definitely not be writable.

Fair enough -- a simpler change might be to poison /etc/passwd and call `su` to a user that has uid 0, since that requires no shell code nor a readable binary, and this seems to have worked in a slightly modified POC:

  f=g.open("/etc/passwd",0);
  e="rkeene:x:0:0:System administrator:/root:/run/current-system/sw/bin/bash\n".encode()
  ...
  g.system("/run/wrappers/bin/su - rkeene")

There is a PoC that does exactly that here: https://github.com/tgies/copy-fail-c

It being readable is the default configuration most places, after all the purpose is to call it from a non-privileged user. But I could see it being made non-readable since its use is discouraged nowadays... though then I'd expect sudo to be readable as an alternative.

My `sudo` is also not readable. Files/directories don't need to be readable to be executed. I can still use `su` and `sudo`.

Oh yeah, duh.

Still every machine I've looked at, and I've since looked at a couple more, has it r-x for world.