SYSAD, TASK 1

Mode: Normal

Prerequisites

sudo must be installed.

The filesystem (and kernel) must support POSIX ACLs. The getfacl and setfacl utilities must be installed. (eg. through the package acl on Debian)

GNU make must be installed.

Installation

Clone the repo, preferably in a clean environment such as a chroot directory or Docker container, and run

make

This will install to /usr/local/bin, and data files go in /usr/share. Building in a chroot is supported through the DESTDIR option.

.bashrc will be updated, and its current state will be saved.

Uninstall

make uninstall

The saved .bashrc will be restored; you have to manually add in any changes you made to your .bashrc since then.

General Info

Improvements

I have improved on the basic instructions in several instances -

• The use of a Makefile for installation, instead of a regular Bash script -

  • allows for easier installation.
  • By adding scripts to PATH, the aliases themselves can just call these scripts. The alternative would be to put all the code in every user's .bashrc, cluttering their configuration files with several hundred lines of code.
  • Automated uninstallation (make uninstall) is also possible.

• The use of ACLs means that the HAD need not be the root user. After all, the root account should not be used unless necessary for system adminstration.

• Numerous small details -

  • Mess allocation updates the mess capacity at the top of the file and clears the list of appended roll numbers, meaning that it can be run multiple times - all the students need not apply first.
  • When updating the defaulter list, the Warden can specify any deadline for fee payment, and each student's payment timestamps are compared against it to see if they have exceeded it.

Limitations

A list of problems I wanted to solve, but could not feasibly do so.

• Bash cannot perform floating-point arithmetic. To avoid requiring another external utility for this purpose, the fee amounts are truncated to ints, sacrificing precision for simplicity.

• A student can remove the ACLs from their home directory and any directory under it, meaning they can prevent the HAD or warden from seeing it. Even if ACLs were not used, this issue would persist; as long as the user has write access to an item, they can change its permissions and/or ACLs.

• The mess.txt file can be edited by anyone; this means that students can tamper with it in many ways, such as filing preferences multiple times, or just directly editing it maliciously. This cannot be mitigated without contradicting the given instructions.

• It is not possible to prevent students from forging transactions on fees.txt, with the current server layout.

  • If the fees.txt file for each student is placed directly inside their home directory, they can easily forge transactions by writing to the file, UNLESS the write-permissions for both the file AND the enclosing directory (their home directory) are removed. However, removing write permissions for the home directory will prevent the student from creating files/directories in their home directory!
  • While in the current case it does not matter, since the transactions are basically meaningless, the fact remains that for a real implementation of transactions, the layout will have to be completely changed.
  • A potential solution, partially implemented here, is to put the fees.txt file in a seperate fees directory - owned by HAD and without write permissions - under each user's home directory. The fee-breakup executable would then need to be owned by HAD and have the SGID special permission set.
    • I have implemented this much; the problem is that the Linux kernel ignores the SUID and SGID bits on non-binary files (because of a historically large number of insecure scripts that were SUID root), meaning that the student user running the script will not get the required permissions. For a working implementation, fee-breakup would have to be a binary file.
    • A possible solution is to compile it to C source code via shc, but this is beyond the scope of this task.

• As such, write permission has been given to the fees directory.