Privilege Coding Example

This section compares how privileges are bracketed using the superuser model and the least privilege model.

Privilege Bracketing in the Superuser Model

The following example demonstrates how privileged operations are bracketed in the superuser model.

Example 2-1  Superuser Privilege Bracketing Example

/* Program start */
uid = getuid();
seteuid(uid);

/* Privilege bracketing */
seteuid(0);
/* Code requiring superuser capability */
...
/* End of code requiring superuser capability */
seteuid(uid);
...
/* Give up superuser ability permanently */
setreuid(uid,uid);

Privilege Bracketing in the Least Privilege Model

This example demonstrates how privileged operations are bracketed in the least privilege model. The example uses the following assumptions:

• The program is setuid 0.

• The permitted and effective sets are initially set to all privileges as a result of setuid 0.

• The inheritable set is initially set to the basic privileges.

• The limit set is initially set to all privileges.

An explanation of the example follows the code listing.

Example 2-2  Least Privilege Bracketing Example

1  #include <priv.h>
2  /* Always use the basic set. The Basic set might grow in future
3   * releases and potentially retrict actions that are currently
4   * unrestricted */
5  priv_set_t *temp = priv_str_to_set("basic", ",", NULL);

6  /* PRIV_FILE_DAC_READ is needed in this example */
7  (void) priv_addset(temp, PRIV_FILE_DAC_READ);

8  /* PRIV_PROC_EXEC is no longer needed after program starts */
9  (void) priv_delset(temp, PRIV_PROC_EXEC);

10 /* Compute the set of privileges that are never needed */
11  priv_inverse(temp);

12  /* Remove the set of unneeded privs from Permitted (and by
13   * implication from Effective) */
14  (void) setppriv(PRIV_OFF, PRIV_PERMITTED, temp);

15  /* Remove unneeded priv set from Limit to be safe */
16  (void) setppriv(PRIV_OFF, PRIV_LIMIT, temp);

17  /* Done with temp */
18  priv_freeset(temp);

19  /* Now get rid of the euid that brought us extra privs */
20  (void) seteuid(getuid());

21  /* Toggle PRIV_FILE_DAC_READ off while it is unneeded */
22  priv_set(PRIV_OFF, PRIV_EFFECTIVE, PRIV_FILE_DAC_READ, NULL);

23  /* Toggle PRIV_FILE_DAC_READ on when special privilege is needed*/
24  priv_set(PRIV_ON, PRIV_EFFECTIVE, PRIV_FILE_DAC_READ, NULL);

25  fd = open("/some/retricted/file", O_RDONLY);

26  /* Toggle PRIV_FILE_DAC_READ off after it has been used */
27  priv_set(PRIV_OFF, PRIV_EFFECTIVE, PRIV_FILE_DAC_READ, NULL);

28  /* Remove PRIV_FILE_DAC_READ when it is no longer needed */
29  priv_set(PRIV_OFF, PRIV_ALLSETS, PRIV_FILE_DAC_READ, NULL);

The program defines a variable that is named temp. The temp variable determines the set of privileges that are not needed by this program. Initially in line 5, temp is defined to contain the set of basic privileges. In line 7, the file_dac_read privilege is added to temp. The proc_exec privilege is necessary to exec(1) new processes, which is not permitted in this program. Therefore, proc_exec is removed from temp in line 9 so that the exec(1) command cannot execute new processes.

At this point, temp contains only those privileges that are needed by the program, that is, the basic set plus file_dac_read minus proc_exec. In line 11, the priv_inverse() function computes the inverse of temp and resets the value of temp to the inverse. The inverse is the result of subtracting the specified set, temp in this case, from the set of all possible privileges. As a result of line 11, temp now contains those privileges that are never needed by the program. In line 14, the unneeded privileges that are defined by temp are subtracted from the permitted set. This removal effectively removes the privileges from the effective set as well. In line 16, the unneeded privileges are removed from the limit set. In line 18, the temp variable is freed, since temp is no longer needed.

This program is aware of privileges. Accordingly, the program does not use setuid and can reset the effective UID to the user's real UID in line 20.

The file_dac_read privilege is turned off in line 22 through removal from the effective set. In a real program, other activities would take place before file_dac_read is needed. In this sample program, file_dac_read is needed for to read a file in line 25. Accordingly, file_dac_read is turned on in line 24. Immediately after the file is read, file_dac_read is again removed from the effective set. When all files have been read, file_dac_read is removed for good by turning off file_dac_read in all privilege sets.

The following table shows the transition of the privilege sets as the program progresses. The line numbers are indicated.

Table 2-2  Privilege Set Transition

Step temp Set Permitted Privilege Set Effective Privilege Set Limit Privilege Set Initially – all all all Line 5 – temp is set to basic privileges basic all all all Line 7 – file_dac_read is added to temp. basic + file_dac_read all all all Line 9 – proc_exec is removed from temp. basic + file_dac_read – proc_exec all all all Line 11 – temp is reset to the inverse. all – (basic + file_dac_read – proc_exec) all all all Line 14 – The unneeded privileges are turned off in the permitted set. all – (basic + file_dac_read – proc_exec) basic + file_dac_read – proc_exec basic + file_dac_read – proc_exec all Line 16 – The unneeded privileges are turned off in the limit set. all – (basic + file_dac_read – proc_exec) basic + file_dac_read – proc_exec basic + file_dac_read – proc_exec basic + file_dac_read – proc_exec Line 18 – The temp file is freed. – basic + file_dac_read – proc_exec basic + file_dac_read – proc_exec basic + file_dac_read – proc_exec Line 22 – Turn off file_dac_read until needed. – basic – proc_exec basic – proc_exec basic + file_dac_read – proc_exec Line 24 – Turn on file_dac_read when needed. – basic + file_dac_read – proc_exec basic + file_dac_read – proc_exec basic + file_dac_read – proc_exec Line 27 – Turn off file_dac_read after read() operation. – basic – proc_exec basic – proc_exec basic + file_dac_read – proc_exec Line 29 – Removefile_dac_read from all sets when no longer needed. – basic – proc_exec basic – proc_exec basic – proc_exec