This is NOT for security purposes.
Suppose I wanted to execute a shell script and, on a best effort basis, assume it left no side effects aside from things like auditd logging, avc reports, syslog, etc. Effectively, I want to drop all write privileges for the current user for the entire shell session. I'm not interested in setting up groups and lower privileged users with carefully managed file permissions. What options exist?
Ideally I'd like to target any posix shell, but I started with bash because it's what I'm most familiar with.
Reading through man bash, restricted shells looked promising but they're too restrictive in the wrong places. e.g. I still want to be able use
cd and it doesn't prevent something like
echo foo | tee /file. Likewise, bash
set options aren't restrictive enough since they don't restrict subprocesses from writing as well, e.g.
(set -C; echo foo | tee /file) doesn't accomplish what I'm looking for.
I'd also like to avoid overlayfs, as I'd like to be able to enter/exit this "read-only" mode in a single SSH session.
The closest I've gotten is dropping the max filesize ulimit to 0:
(ulimit -f 0; echo "blows up w/ signal SIGXFSZ" > file; ); echo $?
This seems pretty close to what I'm looking for and works in shells like
dash, but it still suffers from leaving empty files around. So in the example above "file" would have been created, but would be empty.
I feel like there is a way to confine my user session with SELinux to something more restrictive, but could use a pointer to help.
Any other options?
Another option I'm interested in is simply monitoring any IO write during a process. Perhaps cgroups could be used for this, but I'm unfamiliar with them. Or perhaps systemd-run could be useful too.