This is an exercise from my last Computer Security written exam:

A restaurant processes orders with a FIFO strategy. Orders are stored in file orders.txt.

program enqueue modifies orders.txt by adding an order to the tail of the queue.
program dequeue modifies orders.txt by deleting the order at the head of the queue.

Define a UNIX-based ACL of the files so to ensure that the following access control policy is enforced by the operating system:

willie and winona, the waiters, can add new orders, but cannot read nor delete existing orders
charlie and cathy, the cooks, can read and delete existing orders, but cannot add new orders, nor carry out any other modification to orders.txt.
mario, the maitre must be able to read the orders (and possibly do other actions).

Please provide your answer by completing the dump of the ls -l shell command (assuming that all relevant files are in the current folder):

---------- ________ ________ orders.txt
---------- ________ waiters  enqueue
---------- ________ cooks    dequeue

where cooks is a group containing charlie and cathy and waiters is a group containing willie and winona.

During the exam I wasn't able to answer as precisely as I think I could have. So now I will recreate the environment for this exercise and I will test whether my solution is correct.

my solution

First of all let's write dequeue program, it's a simple C file to append a list to orders.txt

// enqueue.c
#include <stdio.h>
#include <stdlib.h>

int main()
{
  FILE *pFile;

  pFile = fopen("orders.txt", "a");
  if (pFile == NULL)
  {
    perror("Error appending to file.");
    exit;
  }
  fprintf(pFile, "just a simple order");
  fclose(pFile);
}

Dequeue comes next, in order to remove a line I am just overriding the first line with a newline"\n"

// dequeue.c
#include <stdio.h>
#include <stdlib.h>

int main()
{
  FILE *pFile;

  pFile = fopen("orders.txt", "w");
  if (pFile == NULL)
  {
    perror("Error opening file.");
    exit;
  }
  fprintf(pFile, "\\n");
  fclose(pFile);
}

Now let's create all the users and groups (-m creates a system user with a home directory and a login shell), then we compile the two simple c programs as mario and we set some permission bits.

# init.sh
groupadd waiters
groupadd cooks

useradd -m charlie
useradd -m cathy
useradd -m willie
useradd -m winona
useradd -m mario

usermod -a -G waiters willie
usermod -a -G waiters winona

usermod -a -G cooks charlie
usermod -a -G cooks cathy

runuser -u mario mkdir /home/mario/acl
runuser -u mario -- gcc -o /home/mario/acl/enqueue enqueue.c
runuser -u mario -- gcc -o /home/mario/acl/dequeue dequeue.c
runuser -u mario -- touch /home/mario/acl/orders.txt
chgrp waiters /home/mario/acl/enqueue
chgrp cooks /home/mario/acl/dequeue
chmod o-x /home/mario/acl/enqueue
chmod o-r /home/mario/acl/enqueue
chmod u+s /home/mario/acl/enqueue
chmod o-x /home/mario/acl/dequeue
chmod o-r /home/mario/acl/dequeue
chmod u+s /home/mario/acl/dequeue

This is the simple dockerfile I am using to have a clean enviroment, vim was installed for debugging reasons

# Dockerfile
FROM ubuntu:latest
RUN apt-get update && apt-get install -y vim gcc
ADD . .
RUN sh init.sh

Now this is the output of ls -l command:

$: ls -l
-rwsr-x--- 1 mario cooks   16832 Jan 13 00:18 dequeue
-rwsr-x--- 1 mario waiters 16832 Jan 13 00:18 enqueue
-rw-r--r-- 1 mario mario       1 Jan 13 00:19 orders.txt

Let's see if it works as expected by running a few commands:

root@b17f8eed5dcf:/home/mario/acl\\# su willie
$ ./enqueue
$ cat orders.txt
just a simple order
$ ./dequeue
sh: 4: ./dequeue: Permission denied
root@b17f8eed5dcf:/home/mario/acl\\# su cathy
$ ./dequeue
$ cat orders.txt

$ ./enqueue
sh: 3: ./enqueue: Permission denied
root@b17f8eed5dcf:/home/mario/acl\\# su mario
$ echo "i am the maitre, i do whatever i want" > orders.txt
$ cat orders.txt
i am the maitre, i do whatever i want

Yay! Everything works as expected, it turns out the exercise was definitely doable, I am not entirely sure what happened during the exam, however the most important thing is I understand Unix ACLs.
Here you can find a repository with the source code.

unix access control exercise

my solution