[TUHS] Set-uid shell scripts

[jason-tuhs at shalott.net]

> A related problem is the inherent race condition:
> If you do
> 	ln -s /bin/setuidscript .
> 	./setuidscript
> ./setuidscript is opened twice: once when the kernel
> reads it and finds #! as magic number and execs the
> interpreter, a second time when the interpreter opens
> ./setuidscript.  If you meanwhile run something that
> swoops in in the background and replaces ./setuidscript
> with malicious instructions for the interpreter, you
> win.

This was always described to me as the canonical reason why setuid 
interpreted scripts were a security hole, irrespective of any specifics 
in the shell or other interpreter.

However, there's a workaround: use fdescfs.  fdescfs allows the kernel to 
open the script, and then pass the fdescfs path for the (already open) 
descriptor to the interpreter as the command to run.

According to https://www.in-ulm.de/~mascheck/various/shebang/#setuid, this 
is supported by many systems, including Solaris, several BSDs, and OSX 
(with a sysctl).


  -Jason