Shell – Run shell scripts through a website

Linux ignores the setuid¹ bit on all interpreted executables (i.e. executables starting with a #! line). The comp.unix.questions FAQ explains the security problems with setuid shell scripts. These problems are of two kinds: shebang-related and shell-related; I go into more details below.

If you don't care about security and want to allow setuid scripts, under Linux, you'll need to patch the kernel. As of 3.x kernels, I think you need to add a call to install_exec_creds in the load_script function, before the call to open_exec, but I haven't tested.

Setuid shebang

There is a race condition inherent to the way shebang (#!) is typically implemented:

1. The kernel opens the executable, and finds that it starts with #!.
2. The kernel closes the executable and opens the interpreter instead.
3. The kernel inserts the path to the script to the argument list (as argv[1]), and executes the interpreter.

If setuid scripts are allowed with this implementation, an attacker can invoke an arbitrary script by creating a symbolic link to an existing setuid script, executing it, and arranging to change the link after the kernel has performed step 1 and before the interpreter gets around to opening its first argument. For this reason, most unices ignore the setuid bit when they detect a shebang.

One way to secure this implementation would be for the kernel to lock the script file until the interpreter has opened it (note that this must prevent not only unlinking or overwriting the file, but also renaming any directory in the path). But unix systems tend to shy away from mandatory locks, and symbolic links would make a correct lock feature especially difficult and invasive. I don't think anyone does it this way.

A few unix systems (mainly OpenBSD, NetBSD and Mac OS X, all of which require a kernel setting to be enabled) implement secure setuid shebang using an additional feature: the path /dev/fd/N refers to the file already opened on file descriptor N (so opening /dev/fd/N is roughly equivalent to dup(N)). Many unix systems (including Linux) have /dev/fd but not setuid scripts.

1. The kernel opens the executable, and finds that it starts with #!. Let's say the file descriptor for the executable is 3.
2. The kernel opens the interpreter.
3. The kernel inserts /dev/fd/3 the argument list (as argv[1]), and executes the interpreter.

Sven Mascheck's shebang page has a lot of information on shebang across unices, including setuid support.

Setuid interpreters

Let's assume you've managed to make your program run as root, either because your OS supports setuid shebang or because you've used a native binary wrapper (such as sudo). Have you opened a security hole? Maybe. The issue here is not about interpreted vs compiled programs. The issue is whether your runtime system behaves safely if executed with privileges.

• Any dynamically linked native binary executable is in a way interpreted by the dynamic loader (e.g. /lib/ld.so), which loads the dynamic libraries required by the program. On many unices, you can configure the search path for dynamic libraries through the environment (LD_LIBRARY_PATH is a common name for the environment variable), and even load additional libraries into all executed binaries (LD_PRELOAD). The invoker of the program can execute arbitrary code in that program's context by placing a specially-crafted libc.so in $LD_LIBRARY_PATH (amongst other tactics). All sane systems ignore the LD_* variables in setuid executables.

• In shells such as sh, csh and derivatives, environment variables automatically become shell parameters. Through parameters such as PATH, IFS, and many more, the invoker of the script has many opportunities to execute arbitrary code in the shell scripts's context. Some shells set these variables to sane defaults if they detect that the script has been invoked with privileges, but I don't know that there is any particular implementation that I would trust.

• Most runtime environments (whether native, bytecode or interpreted) have similar features. Few take special precautions in setuid executables, though the ones that run native code often don't do anything fancier than dynamic linking (which does take precautions).

• Perl is a notable exception. It explicitly supports setuid scripts in a secure way. In fact, your script can run setuid even if your OS ignored the setuid bit on scripts. This is because perl ships with a setuid root helper that performs the necessary checks and reinvokes the interpreter on the desired scripts with the desired privileges. This is explained in the perlsec manual. It used to be that setuid perl scripts needed #!/usr/bin/suidperl -wT instead of #!/usr/bin/perl -wT, but on most modern systems, #!/usr/bin/perl -wT is sufficient.

Note that using a native binary wrapper does nothing in itself to prevent these problems. In fact, it can make the situation worse, because it might prevent your runtime environment from detecting that it is invoked with privileges and bypassing its runtime configurability.

A native binary wrapper can make a shell script safe if the wrapper sanitizes the environment. The script must take care not to make too many assumptions (e.g. about the current directory) but this goes. You can use sudo for this provided that it's set up to sanitize the environment. Blacklisting variables is error-prone, so always whitelist. With sudo, make sure that the env_reset option is turned on, that setenv is off, and that env_file and env_keep only contain innocuous variables.

TL,DR:

• Setuid shebang is insecure but usually ignored.
• If you run a program with privileges (either through sudo or setuid), write native code or perl, or start the program with a wrapper that sanitizes the environment (such as sudo with the env_reset option).

¹ _{This discussion applies equally if you substitute “setgid” for “setuid”; they are both ignored by the Linux kernel on scripts}

I assume you can run the script with: ./script1.sh (which start the script in a shell) instead of . script1.sh (which will source them in your current shell) ?

If so:

#!/bin/bash

./script1.sh   &&
{ ssh user@remote "cd /path/to/  && script2.sh" ; }  &&
./script3.sh   &&
./script4.sh

&& ensure that the following instructions is executed ONLY if the previous returned "0" (="ok") (so make sure your script does return the proper "0" if OK, and something else (a small positive integer, such as "1") if NOT OK).

(Note that you can have && at the end of a line, as I did here, and the following thing on the next line, without the need to \ the newline in-between. "&&" at the end of the line tells bash that the line can be continued on the following line. [thank to @Dennis for correcting me: I thought it also worked to not put a '\newline' when "&&" was on the next line, which is more readable... but doesn't work. If you want "&&" on the next line, you need to backslash the previous newline])