Sample Midterm

  1. Here is a fragment of code from a program that reads data from a file into a dynamically allocated part of memory. There are at least 3 things in this code that make it very non-robust. Find any 3, say why each is a (potential) problem, and how you would fix each. (This question asks about robustness, not commenting style - the comments are just there to help you figure out what is going on.) /* read nchars characters from the file named filename */
    /* and put them into dynamically allocated memory      */
    char *load(int nchars, char *filename)
        char *p;        /* pointer to allocated memory */
        FILE *fp    ;    /* pointer to the opened file */
        /* allocate space for nchars char */
        p = malloc(nchars * sizeof(char));
        /* open the file */
        fp = fopen(filename, "r");
        /* read nchars characters from the file that   */
        /* fp points to, and put it in the memory that */
        /* begins at address p                         */
        (void) fread(p, sizeof(char), nchars, fp);
        /* close the file */
        (void) fclose(fp);
        /* return the address of the allocated memory */
  2. Why is a precise statement of security requirements critical to the determination of whether a given system is secure?
  3. Which of the following demonstrate violations of the principle of least privilege? Please justify your answer.
    1. The UNIX root account?
    2. A user whose function is to maintain and install system software. This user has access to the source files and directories, access to only those programs needed to build and maintain software, and can copy executables into system directories for other users. This user has no other special privileges.
  4. Consider the Bell-LaPadula multilevel security model. If a subject with security label (L, C) can read an object with security label (L, C), then (L, C) is said to dominate (L, C). Prove that this dominates relation is reflexive, antisymmetric, and transitive.
  5. Define each of the following terms in one short sentence:
    1. public key cryptosystem
    2. separation of duty
    3. ciphertext
    4. constrained data item
    5. principle of fail-safe defaults


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