localapps whitelist options

<Unspecified individual> wants the ability to lock down LTSP hybrid 
clients to run only specified applications with ltsp-localappsd.  I 
believe upstream might be good with an optional whitelist that is 
disabled by default.  Vagrant agrees.

OPTION 1: ALLOW ONLY SINGLE COMMANDS
====================================
LOCAL_APPS_WHITELIST="/usr/bin/xterm /usr/bin/gcalctool"
# This option comes from lts.conf.
for cmd in $LOCAL_APPS_WHITELIST; do
     if [ "$cmd" = "$LTSP_COMMAND" ]; then
         echo "Executing $cmd"
     else
         echo "Rejecting $LTSP_COMMAND"
     fi
done

This would be the easiest to implement and confirm that it works in a 
secure fashion.  The key drawback however is it is impossible to run 
multi-word commands like:

ltsp-localapps /usr/bin/gimp /home/username/test.jpg

Perhaps this is not bad, because you can do this in the default 
configuration where any application is allowed to be executed.

OPTION 2: PASS ALL OPTIONS TO A POLICY ENFORCEMENT SCRIPT
=========================================================
Alternatively, a lts.conf option could direct ltsp-localappsd to pass 
the entire command line to an arbitrary wrapper script.  That script 
internally decides if something should be executed or not.

ltsp-localapps /usr/bin/gimp /home/username/test.jpg
(received by ltsp-localappsd)
(executes)
/usr/bin/arbitrary-localapps-wrapper /usr/bin/gimp /home/username/test.jpg
(internally decides it is OK to execute this)
(runs it)

Any opinions about which path we should take, or any better ideas?  I'm 
leaning towards #1 because it is far simpler and self-contained.  Or 
perhaps we do #2, with #1 as the simple provided policy wrapper.  The 
deployer can decide to replace the wrapper script?

Warren Togami
[email protected]

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