== What you can't do before you drop setuid permissions

Let us suppose that you have a program that is setuid root but that
usually drops its setuid status and reverts to running as the user that
ran it. Of course you want to do this as early as possible to reduce
the potential security risks, but at the same time you might want do
some operations while still root (for reasons of either necessity or
convenience in your code structure).

Consider the following tempting sequence, presented in pseudo-code:

> ((chdir(pw->pw_dir);)) \\
> _drop-setuid();_

Innocently, one would think that this code is harmless and safe. One
would be mistaken; this code is wrong and buggy. If the user's home
directory is on NFS and has restricted access permissions, the _chdir()_
will fail, because ~~root has less permissions than a normal user when
working over NFS~~.

Now, consider this sequence (and assume that _$HOME_ is secure,
since this is just pseudo-code being written for clarity):

> _open($HOME/.config, "r");_ \\
> _drop-setuid();_

This code is not just buggy, it is a security exposure. It is buggy
for the same reason that the first code is (root may not be able to
open _$HOME/.config_, although the user could), and it is a security
exposure because the user can make your program open anything on the
system with root permissions. Even assuming that there is no way at all
to make your program print out any part of the contents of the user's
(apparent) configuration file (either correctly formatted or full of
syntax errors), there are things that cause side effects when opened and
closed.

(And [[you cannot fix this ../programming/CheckThenUseIsDangerous]].)

I don't know what you can safely do before dropping setuid permissions;
it's a dangerous and hard problem that I don't know enough about to
have an opinion beyond 'as little as possible'. But I do know that you
definitely can't do either of these things.

(Now you can probably guess why I was [[tracing a setuid Linux program
../linux/SystemTapSetuidTracing]], and why I redacted the program name.)