I'm going over a practice question and I have trouble understanding what it's asking.

Draw an access control matrix for the following Linux file system. Group A1 contains S1 and S3, group A2 contains S3. Only show users as subjects.

file system

So is the answer to the question supposed to look like


What is the answer to the question?


So to summarize:

  • Group 1 contains S1 and S3
  • Group 2 contains S3

Then it is pretty simple, if you know that in Linux, the 9-character permission scheme can be divided as follows:

  • First three characters are the owner's permissions
  • Next three characters are the group's permissions
  • Last three characters are the permissions of all of the other users

Then, following the assumption I made, you know the following:

Permissions    a     b    c
S1             rwx   -    rwx (S1 is member of group 1: group 1 has rwx permission)
S2             rwx   rwx  - (due to explicit owner deny on the last row of the table)
S3             rwx   -    rwx (due to group inherited allow permission of group 1)
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  • From the diagram (ls -la) output is it possible to infer what group S2 belongs to? – Celeritas Dec 9 '15 at 11:04
  • @Celeritas, sorry, I'm not sure what you mean by this? What is the (ls - la) output? – Michael Dec 9 '15 at 12:18
  • Basically I'm asking, can you explain why you think the question is not consistent? – Celeritas Dec 10 '15 at 2:28
  • I'm asking, from the first diagram, is it possible to infer which users are in which group (e.g. somehow using process of elimination)? – Celeritas Dec 10 '15 at 2:29
  • That's what I mentioned in my answer (above the horizontal line): ////. You say that: Group 1 contains S1 and S3; Group 2 contains S3. /// From the image, it shows that:Group 1 contains S1 and S2;Group 2 contains S1////From the solution, it shows that: S3 is in group 2 – Michael Dec 10 '15 at 6:47

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