实现Linux下的Mandatory Access Control机制，对文件进行访问控制管理。

Mandatory Access Control (MAC)

In this homework assignment, you will implement the read-down, write-up mandatory access control policy discussed in class. Specifically, you will write a command-line tool called mac , which, together with setuid / setgid functionalities in Linux, is used as a mechanism to implement this policy.

• This command line tool mac will be owned by the user root and the group root , and be given the permissions: 6755 , where the setuid , setgid , and executable bit are set as follows.
• Objects: There are four documents (a document for each security classification level) that are protected by the mandatory access control policy: top\_secret.data , secret.data , confidential.data , and unclassified.data . These four files are classified as TOP\_SECRET , SECRET , CONFIDENTIAL , and UNCLASSIFIED , respectively. These four files will be (i) pre-created, (ii) owned by the user root and the group root , and (iii) given the permissions: 0640 as follows.
  Note that the owners and permissions of these files must stay the same at all times.
• Subjects: All non-root users in the system are considered as subjects. Security clearance levels of subjects are specified in a file called mac.policy , which is owned by the user root and the group root , and given the permissions: 0640 as follows.
  Remember that subjects whose access is controlled by our policy are non-root users that is, the command line tool mac will be executed by a non-root user whose primary group is the same as the name of that user. (e.g., the user david and the group david ) This effectively prevents subjects from reading or manipulating the data files directly this requires accesses to the data files be always mediated by the program mac .
• Dropping the root privilege: During the execution of mac , drop the root privilege when you do not need it anymore, by changing the effective uid and gid of the process. (Hint: Use the following functions: seteuid , getuid , setegid , and getgid .)
• Logging: After dropping your root privilege, append the command line arguments followed by a newline character to the per-user log file named [username].log . Do not log, however, what has been writen.
  $ ./mac read secret.data
  $ cat david.log read secret.data
  $ ./mac write secret.data TEST\_INPUT
  $ cat david.log read secret.data write secret.data

If the log file doesn’t exist for the given user, it must be created before appending. Once created, this file must at all times be owned by the executing non- root user and group, and be given the permissions: 0640 . You can assume that the user has the permission to create a file in its current working directory. (Hint: Use the following functions: getpwuid and umask .)

Interface

All variable inputs to the program (e.g., [document file] ) will be [a-zA-Z0-9\_-.] (i.e., alphanumeric, underscore, dash, and period). All matching is case-sensitive.
You must implement the following command-line interface:

• ./mac read
  reads and prints the content of the document file (e.g., top\_secret.data ) followed by a newline character on standard output, if and only if (i) the security clearance level for the executing user is specified in mac.policy , and (ii) our read-down confidentiality policy allows it. Otherwise, print ACCESS DENIED , followed by a newline character. You can assume that [document file] is always valid (one of the four files specified above).
• ./mac write
  appends [data] followed by a newline character to the file [document file] , if and only if the executing user has a security clearance level, and the policy allows it. Otherwise, print ACCESS DENIED , followed by a newilne character. You can assume that a given [document file] (one of the four given above) is always valid.

Example

With david:SECRET written in the mac.policy file:

$ whoami
david
$ sudo cat mac.policy
david:SECRET

Running the program as follows:

./mac read secret.data

• will produce the following output on standard output:

THIS IS SECRET DATA

• ./david.log looks like:

read secret.data

And running the program again as follows:

./mac read confidential.data

• will produce the following output on standard output:

THIS IS CONFIDENTIAL DATA

• ./david.log now looks like: read secret.data read confidential.data

And running the program again as follows:

./mac read top_secret.data

• will produce the following output on standard output:

ACCESS DENIED

• ./david.log now looks like:

read secret.data
read confidential.data
read top_secret.data

Implementation

• Your program must work on Ubuntu 22.04 64-bit with the default packages installed. In addition to the default packages, the following packages for languages are also installed:

  • C (gcc)
  • Rust and Cargo (rustc and cargo) with selection of crates pre-installed for you in the grading environment. If you are using Rust, provided Cargo.toml should not be modified.

Again, you’re probably better off if you set up a virtual machine to work on the course assignments early on. You can use VirtualBox, a free and open-source VM monitor software. Or, if you are using MS Windows, you may want to use WSL (WSL version 2 is recommended.) (Ubuntu 22.04 on Microsoft Store).

Submission Instructions.

Submit on LearnUs (ys.learnus.org) your source code, along with a Makefile and README . When the command make is run, the Makefile must create your executable, called mac , on the same directory as your Makefile and README . These files must not be included in any subdirectory. Note that we may invoke make multiple times, and it needs to work every single time. After creating mac , we will change its owner and owning group, as well as permissions, as specified above. We will also create the data files as well as the policy file, and change their owner and owning group and permissions as specified above. Your README file must be plain text and should contain your name, student ID, and a description of how your program works. Your submission can be zipped, if required your submission will be unzipped once before grading. However, the directory structure described above still apply.

Grading Rubric

• All required files exist, and make successfully creates an executable file mac.
• Produces a log file in a correct format.
• The log file’s owner/group owner/permissions are correct at all times.
• Read accesses are correctly controlled.
• Write accesses are correctly controlled.

Note

It is your responsibility to comply with the specification as well as our grading environment. You can request regrading for minor problems (e.g., issues caused by using different environments), but they will result in deductions of 1 or more points.