Chapter 09: Process Credentials

Exercise 9-1

Question

Assume in each of the following cases that the initial set of process user IDs is real=1000 effective=0 saved=0 file-system=0. What would be the state of the user IDs after the following calls? [in answers]

Answer

A few notes to frame the answers here: the effective userID is root but the real is not (e.g. sudo or similar).

(a) setuid(2000);

  real=2000, effective=2000, saved=2000, file-system=2000

(b) setreuid(-1, 2000);

  real=1000, effective=2000, saved=2000, file-system=2000

(c) seteuid(2000);

  real=1000, effective=2000, saved=0, file-system=2000

(d) setfsuid(2000);

  real=1000, effective=0, saved=0, file-system=2000

(e) setresuid(-1, 2000, 3000);

  real=1000, effective=2000, saved=3000, file-system=2000

Exercise 9-2

Question

Is a process with the following user IDs privileged? Explain your answer?

real=0, effective=1000, saved=1000, file-system=1000

Answer

"Not really." Since the effective userID is not privileged, the process is not privileged. That being said, it could easily be made privileged again via system calls that allow one to set the effective uid back to the real uid.

Exercise 9-3

Question

Implement initgroups() using setgroups() and library functions for retrieving information from the password and group files (Section 8.4). Remember that a process must be privileged in order to be able to call setgroups().

Answer

See initgroups.c.

Exercise 9-4

Question

If a process whose user IDs all have the value X execute a set-user-ID program whose ID, Y, is nonzero, then thep rocess credential are set as follows:

real=X effective=Y saved=Y

(We ignore the file-system user ID, since it tracks the effective user ID.) Show the setuid(), seteuid(), setreuid(), and setresuid() calls, respsectively, that would be used to perform the follwoing operations:

a) Suspend and resume the set-user-ID identity (i.e., switch the effective user ID to the value of the real user ID and then back to the saved set-user-ID).

b) Permanently drop the set-user-ID identity (i.e., ensure that the effective user ID and the saved set-user-ID are set to the value of the real user ID).

(This exercise also requires the use of getuid() and geteuid() to retrieve theh process's real and effective user IDs.)_ Note that for certain of the system calls listed above, some of these operations can't be performed.

Answer

eff_uid = geteuid();

// setuid()
setuid(getuid()); /* suspend */
setuid(eff_uid); /* resume */
/* dropping permanently not possible */

// seteuid()
seteuid(getuid()); /* suspend */
seteuid(eff_uid); /* resume */
/* dropping permanently not possible */

// setreuid()
setreuid(-1, geteuid()); /* suspend */
setreuid(-1, eff_uid); /* resume */
setreuid(getuid(), getuid()) /* drop permanently */

// setresuid()
setresuid(-1, getuid(), -1); /* suspend */
setresuid(-1, eff_uid, -1); /* resume */
setresuid(getuid(), getuid(), getuid()); /* drop permanently */

Exercise 9-5

Question

Repeat the previous exercise for a process executing a set-user-ID-root program, which has the following initial set of process cerdentials:

real=X effective=0 saved=0

Answer

// setuid()
/* suspend not possible */
/* resume not possible */
/* dropping permanently not possible */

// seteuid()
seteuid(getuid()); /* suspend */
seteuid(0); /* resume */
/* dropping permanently not possible */

// setreuid()
setreuid(-1, geteuid()); /* suspend */
setreuid(-1, 0); /* resume */
setreuid(getuid(), getuid()) /* drop permanently */

// setresuid()
setresuid(-1, getuid(), -1); /* suspend */
setresuid(-1, 0, -1); /* resume */
setresuid(getuid(), getuid(), getuid()); /* drop permanently */