pkexec: enforce absolute shell paths

[stoeckmann]

Reading /etc/shells file directly has the effect that comments are parsed as well. If a user sets environment variable SHELL to a value which matches one of these comments, it is passed through pkexec.

The shadow tools would not allow such a login shell, so be as strict as shadow when it comes to parsing /etc/shell.

Proof of Concept:

1. Add a comment to /etc/shells (if it does not already exist)

# Begin /etc/shells

/bin/sh
/bin/bash

# End /etc/shells

2. For sake of completeness, try to add a comment line through latest chsh of shadow:

$ chsh -s '# Begin /etc/shells'
chsh: # Begin /etc/shells is an invalid shell

3. Run pkexec with SHELL being one of the comment lines

$ SHELL="# Begin /etc/shells" pkexec

You can see:

==== AUTHENTICATING FOR org.freedesktop.policykit.exec ====
Authentication is needed to run `/bin/bash' as the super user
Authenticating as: root
Password:

What should be seen:

The value for the SHELL variable was not found in the /etc/shells file

This incident has been reported.