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Chapter 2 Developing Privileged ApplicationsThis chapter describes how to develop privileged applications. The chapter covers the following topics: Privileged ApplicationsA privileged application is an application that can override system controls and check for specific user IDs (UIDs), group IDs (GIDs), authorizations, or privileges. These access control elements are assigned by system administrators. For a general discussion of how administrators use these access control elements, see Chapter 8, Using Roles and Privileges (Overview), in System Administration Guide: Security Services. The Solaris OS provides developers with two elements that enable a finer-grained delegation of privileges:
The difference between authorizations and privileges has to do with the level at which the policy of who can do what is enforced. Privileges are enforced at the kernel level. Without the proper privilege, a process cannot perform specific operations in a privileged application. Authorizations enforce policy at the user application level. An authorization might be required for access to a privileged application or for specific operations within a privileged application. About PrivilegesA privilege is a discrete right that is granted to a process to perform an operation that would otherwise be prohibited by the Solaris OS. Most programs do not use privileges, because a program typically operates within the bounds of the system security policy. Privileges are assigned by an administrator. Privileges are enabled according to the design of the program. At login or when a profile shell is entered, the administrator's privilege assignments apply to any commands that are executed in the shell. When an application is run, privileges are turned on or turned off programmatically. If a new program is started by using the exec(1) command, that program can potentially use all of the parent process's inheritable privileges. However, that program cannot add any new privileges. How Administrators Assign PrivilegesSystem administrators are responsible for assigning privileges to commands. For more information on privilege assignment, see Privileges (Overview) in System Administration Guide: Security Services. How Privileges Are ImplementedEvery process has four sets of privileges that determine whether a process can use a particular privilege:
Permitted Privilege SetAll privileges that a process can ever potentially use must be included in the permitted set. Conversely, any privilege that is never to be used should be excluded from the permitted set for that program. When a process is started, that process inherits the permitted privilege set from the parent process. Typically at login or from a new profile shell, all privileges are included in the initial set of permitted privileges. The privileges in this set are specified by the administrator. Each child process can remove privileges from the permitted set, but the child cannot add other privileges to the permitted set. As a security precaution, you should remove those privileges from the permitted set that the program never uses. In this way, a program can be protected from using an incorrectly assigned or inherited privilege. Privileges that are removed from the permitted set are automatically removed from the effective set. Inheritable Privilege SetAt login or from a new profile shell, the inheritable set contains the privileges that have been specified by the administrator. These inheritable privileges can potentially be passed on to child processes after an exec(1) call. A process should remove any unnecessary privileges to prevent these privileges from passing on to a child process. Often the permitted and inheritable sets are the same. However, there can be cases where a privilege is taken out of the inheritable set, but that privilege remains in the permitted set. Limit Privilege SetThe limit set enables a developer to control which privileges a process can exercise or pass on to child processes. A child process and the descendant processes can only obtain privileges that are in the limit set. When a setuid(0) function is executed, the limit set determines the privileges that the application is permitted to use. The limit set is enforced at exec(1) time. Removal of privileges from the limit set does not affect any other sets until the exec(1) is performed. Effective Privilege SetThe privileges that a process can actually use are in the process's effective set. At the start of a program, the effective set is equal to the permitted set. Afterwards, the effective set is either a subset of or is equal to the permitted set. A good practice is to reduce the effective set to the set of basic privileges. The basic privilege set, which contains the core privileges, is described in Privilege Categories. Remove completely any privileges that are not needed in the program. Toggle off any basic privileges until that privilege is needed. For example, the file_dac_read privilege, enables all files to be read. A program can have multiple routines for reading files. The program turns off all privileges initially and turns on file_dac_read, for appropriate reading routines. The developer thus ensures that the program cannot exercise the file_dac_read privilege for the wrong reading routines. This practice is called privilege bracketing. Privilege bracketing is demonstrated in Privilege Coding Example. Compatibility Between the Superuser and Privilege ModelsTo accommodate legacy applications, the implementation of privileges works with both the superuser and privilege models. This accommodation is achieved through use of the PRIV_AWARE flag, which indicates that a program works with privileges. The PRIV_AWARE flag is handled automatically by the operating system. Consider a child process that is not aware of privileges. The PRIV_AWARE flag for that process would be false. Any privileges that have been inherited from the parent process are available in the permitted and effective sets. If the child sets a UID to 0, the process's effective and permitted sets are restricted to those privileges in the limit set. The child process does not gain full superuser powers. Thus, the limit set of a privilege-aware process restricts the superuser privileges of any non-privilege-aware child processes. If the child process modifies any privilege set, then the PRIV_AWARE flag is set to true. Privilege CategoriesPrivileges are logically grouped on the basis of the scope of the privilege, as follows:
See the privileges(5) man page for a complete list of the Solaris privileges with descriptions. Note – Solaris provides the zones facility, which lets an administrator set up isolated environments for running applications. See zones(5). Since a process in a zone is prevented from monitoring or interfering with other activity in the system outside of that zone, any privileges on that process are limited to the zone as well. However, if needed, the PRIV_PROC_ZONE privilege can be applied to processes in the global zone that need privileges to operate in non–global zones. Programming with PrivilegesThis section discusses the interfaces for working with privileges. To use the privilege programming interfaces, you need the following header file. #include <priv.h> An example demonstrating how privilege interfaces are used in a privileged application is also provided. Privilege Data TypesThe major data types that are used by the privilege interfaces are:
Privilege InterfacesThe following table lists the interfaces for using privileges. Descriptions of some major privilege interfaces are provided after the table. Table 2–1 Interfaces for Using Privileges
setppriv(): for Setting PrivilegesThe main function for setting privileges is setppriv(), which has the following syntax: int setppriv(priv_op_t op, priv_ptype_t which, \ const priv_set_t *set); op represents the privilege operation that is to be performed. The op parameter has one of three possible values:
which specifies the type of privilege set to be changed, as follows:
set specifies the privileges to be used in the change operation. In addition, a convenience function is provided: priv_set(). priv_str_to_set() for Mapping PrivilegesThese functions are convenient for mapping privilege names with their numeric values. priv_str_to_set() is a typical function in this family. priv_str_to_set() has the following syntax: priv_set_t *priv_str_to_set(const char *buf, const char *set, \ const char **endptr); priv_str_to_set() takes a string of privilege names that are specified in buf. priv_str_to_set() returns a set of privilege values that can be combined with one of the four privilege sets. **endptr can be used to debug parsing errors. Note that the following keywords can be included in buf:
Privilege Coding ExampleThis section compares how privileges are bracketed using the superuser model and the least privilege model. Privilege Bracketing in the Superuser ModelThe 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 ModelThis example demonstrates how privileged operations are bracketed in the least privilege model. The example uses the following assumptions:
An explanation of the example follows the code listing. Note – The source code for this example is also available through the Sun download center. See http://www.sun.com/download/products.xml?id=41912db5. Example 2–2 Least Privilege Bracketing Example1 #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
Guidelines for Developing Privileged ApplicationsThis section provides the following suggestions for developing privileged applications:
About AuthorizationsAuthorizations are stored in the /etc/security/auth_attr file. To create an application that uses authorizations, take the following steps:
Example 2–3 Checking for AuthorizationsThe following code snippet demonstrates how the chkauthattr() function can be used to check a user's authorization. In this case, the program checks for the solaris.job.admin authorization. If the user has this authorization, the user is able to read or write to other users' files. Without the authorization, the user can operate on owned files only. /* Define override privileges */
priv_set_t *override_privs = priv_allocset();
/* Clear privilege set before adding privileges. */
priv_set(PRIV_OFF, PRIV_EFFECTIVE, PRIV_FILE_DAC_READ,
priv_FILE_DAC_WRITE, NULL);
priv_addset(override_privs, PRIV_FILE_DAC_READ);
priv_addset(override_privs, PRIV_FILE_DAC_WRITE);
if (!chkauthattr("solaris.jobs.admin", username)) {
/* turn off privileges */
setppriv(PRIV_OFF, PRIV_EFFECTIVE, override_privs);
}
/* Authorized users continue to run with privileges */
/* Other users can read or write to their own files only */
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