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This chapter is from the book

This chapter is from the book

31.4 Refinement and Implementation

This section focuses on the access control module of the program. We refine the high-level design presented in the preceding section until we produce a routine in a programming language.

31.4.1 First-Level Refinement

Rather than use any particular programming language, we first implement the module in pseudocode. This requires two decisions. First, the implementation language will be block-structured, like C or Java, rather than functional, like Scheme or ML. Second, the environment in which the program will function will be a UNIX-like system such as FreeBSD or Linux.

The basic structure of the security module is

boolean accessok(role rname, command cmd);
    status ← false
    user ← obtain user ID
    timeday ← obtain time of day
    entry ← obtain entry point (terminal line, remote host)
    open access control file
    repeat
      currecord ← obtain next record from file; EOF if none
      if currecord ≠ EOF then
          status ← match(currecord, rname, cmd, user,
                          timeday, entry)
    until currecord = EOF or status = true
    close access control file
    return status

We now verify that this sketch matches the design. Clearly, the interface is unchanged. The variable status will contain the status of the access control file check, becoming true when a match is found. Initially, it is set to false (deny access) because of the principle of fail-safe defaults. If status were not set, and the access control file were empty, status would never be set and the returned value would be undefined.

The next three lines obtain the user ID, the current time of day, and the system entry point. The following line opens the access control file.

The routine then iterates through the records of that file. The iteration has two requirements—that if any record allows access, the routine is to return true, and that if no record grants access, the routine is to return false. From the structure of the file, one cannot create a record to deny access. By default, access is denied. Entries explicitly grant access. So, iterating over the records of the file either produces a record that grants access (in which case the match routine returns true, terminating the loop and causing accessok to return with a value of true) or produces no such record. In that case, status is false, and currecord is set to EOF when the records in the access control file are exhausted. The loop then terminates, and the routine returns the value of status, which is false. Hence, this pseudocode matches the design and, transitively, the requirements.

31.4.2 Second-Level Refinement

Now we will focus on mapping the pseudocode above to a particular language and system. The C programming language is widely available and provides a convenient interface to UNIX-like systems. Given that our target system is a UNIX-like system, C is a reasonable choice. As for the operating system, there are many variants of the UNIX operating system. However, they all have fundamental similarities. The Linux operating system will provide the interfaces discussed below, and they work on a wide variety of UNIX systems.

On these systems, roles are represented as normal user accounts. The root account is really a role account,13 for example. Each user account has two distinct representations of identity:14 an internal user type uid_t,15 and a string (name). When a user specifies a role, either representation may be used. For our purposes, we will assume that the caller of the accessok routine provides the uid_t representation of the role identity. Two reasons make this representation preferable. First, the target systems are unable to address privilege in terms of names, because, within the kernel, process identity is always represented by a uid_t. So the routines will need to do the conversion anyway. The second reason is more complex. Roles in the access control file can be represented by numbers or names. The routine for reading the access control file records will convert the roles to uid_ts to ensure consistency of representation. This also allows the input routine to check the records for consistency with the system environment. Specifically, if the role name refers to a nonexistent account, the routine can ignore the record. So any comparisons would require the role from the interface to be converted to a uid_t.

This leads to a design decision: represent all user and role IDs as integers internally. Fortunately, none of the design decisions discussed so far depend on the representation of identity, so we need not review or change our design.

Next, consider the command. On the target system, a command consists of a program name followed by a sequence of words, which are the command-line arguments to the command. The command representation is an array of strings, in which the first string is the program name and the other strings are the command-line arguments.

Putting this all together, the resulting interface is

int accessok(uid_t rname, char *cmd[])

Next comes obtaining the user ID. Processes in the target system have several identities, but the key ones are the real UID (which identifies the user running the process) and the effective UID (which identifies the privileges with which the process runs).16 The effective UID of this program must have root privileges (see Exercise 4) regardless of who runs the process. Hence, it is useless for this purpose. Only the real UID identifies the user running the program. So, to obtain the user ID of the user running the program, we use

userid = getuid();

The time of day is obtained from the system and expressed in internal format. The internal representation can be given in seconds since a specific date and time (the epoch)17 or in microseconds since that time. It is unlikely that times will need to be specified in microseconds in the access control file. For both simplicity of code and simplicity of the access control data,18 the internal format of seconds will be used. So, to obtain the current time, we use

timeday = time(NULL);

Finally, we need to obtain the location. There is no simple method for obtaining this information, so we defer it until later by encapsulating it in a function. This also localizes any changes should we move this program to a different system (for example, the methods used on a Linux system may differ from those used on a FreeBSD system).

entry = getlocation();

Opening the access control file for reading is straightforward:

if ((fp = fopen(acfile, "r")) == NULL){
    logerror(errno, acfile);
    return(0);
}

Notice first the error checking, and the logging of information on an error. The variable errno is set to a code indicating the nature of the error. The variable acfile points to the access control file name. The processing of the access control records follows:

do {
    acrec = getnextacrec(fp);
    if (acrec != NULL)
        status = match(acrec, rname, cmd, user, timeday,
                 entry);
} while (acrec == NULL || status == 1);

Here, we read in the record—assuming that any records remain—and check the record to see if it allows permission. This looping continues until either some record indicates that permission is to be given or all records are checked. The exact internal record format is not yet specified; hence, the use of functions. The routine concludes by closing the access control file and returning status:

(void) fclose(fp);
return(status);

31.4.3 Functions

Three functions remain: the function for obtaining location, the function for getting an access control record, and the function for checking the access control record against the information of the current process. Each raises security issues.

31.4.3.1 Obtaining Location

UNIX and Linux systems write the user’s account name, the name of the terminal on which the login takes place, the time of login, and the name of the remote host (if any) to the utmp file. Any process may read this file. As each new process runs, it may have an associated terminal. To determine the utmp record associated with the process, a routine may obtain the associated terminal name, open the utmp file, and scan through the record to find the one with the corresponding terminal name. That record contains the name of the host from which the user is working.

This approach, although clumsy, works on most UNIX and Linux systems. It suffers from two problems related to security.

  1. If any process can alter the utmp file, its contents cannot be trusted. Several security holes have occurred because any process could alter the utmp file [2254].

  2. A process may have no associated terminal. Such a detached process must be mapped into the corresponding utmp record through other means. However, if the utmp record contains only the information described above, this is not possible because the user may be logged into multiple terminals. The issue does not arise if the process has an associated terminal, because only one user at a time may be logged into a terminal.

In the first case, we make a design decision that if the data in the utmp file cannot be trusted because any process can alter that file, we return a meaningless location. Then, unless the location specifier of the record allows access from any location, the record will not match the current process information and will not grant access. A similar approach works if the process does not have an associated terminal.

The outline of this routine is

hostname getlocation()
    status ← false
    myterm ← name of terminal
    obtain access control list for utmp
    if any user other than root can alter it then
        return "*nowhere*"
    open utmp
    repeat
       term ← obtain next entry from utmp; otherwise EOF
       if term ≠ EOF and myterm = term then
               status ← true
    until term = EOF or status = true
    if host field of utmp entry = empty
       host = "localhost"
    else
       host = host field of utmp entry
    close utmp
    return host

We omit the implementation due to space limitations.

31.4.3.2 The Access Control Record

The format of the records in the access control file affects both the reading of the file and the comparison with the process information, so we design it here.

Our approach is to consider the match routine first. Four items must be checked: the user name, the location, the time, and the command. Consider these items separately.

The user name is represented as an integer. Thus, the internal format of the user field of the access control record must contain either integers or names that the match routine can convert to integers. If a match occurs before all user names have been checked, then the program needs to convert no more names to integers. So, we adopt the strategy of representing the user field as a string read directly from the file. The match routine will parse the line and will use lazy evaluation to check whether or not the user ID is listed.

A similar strategy can be applied to the location and the set of commands in the record.

The time is somewhat different, because in the previous two cases, the process user ID and the location had to match one of the record entries exactly. However, the time does not have to do so. Time in the access control record is (almost always) a range. For example, the entry “May 30” means any time on the date of May 30. The day begins at midnight and ends at midnight, 24 hours later. So, the range would be from May 30 at midnight to May 31 at midnight, or in internal time (for example) between 1527638400 and 1527724800. In those rare cases in which a user may assume a role only at a precise second, the range can be treated as having the same beginning and ending points. Given this view of time as ranges, checking that the current time falls into an acceptable range suggests having the match routine parse the times and checking whether or not the internal system time falls in each range as it is constructed.

This means that the routine for reading the record may simply load the record as a sequence of strings and let the match routine do the interpretation. This yields the following structure:

record
    role rname
    string userlist
    string location
    string timeofday
    string commands[]
    integer numcommands
end record;

The commands field is an array of strings, each command and argument being one string, and numcommands containing the number of commands.

Given this information, the function used to read the access control records, and the function used to match them with the current process information, are not hard to write, but error handling does deserve some mention.

31.4.3.3 Error Handling in the Reading and Matching Routines

Assume that there is a syntax error in the access control file. Perhaps a record specifies a time incorrectly (for example, “Thurxday”), or a record divider is garbled. How should the routines handle this?

The first observation is that they cannot ignore the error. To do so violates basic principles of security (specifically, the principle of least astonishment19). It also defeats the purpose of the program, because access will be denied to users who need it.20 So, the program must produce an indication of error. If it is printed, then the user will see it and should notify the system administrator maintaining the access control file. Should the user forget, the administrator will not know of the error. Hence, the error must be logged. Whether or not the user should be told why the error has occurred is another question. One school of thought holds that the more information users have, the more helpful they will be. Another school holds that information should be denied unless the user needs to know it, and in the case of an error in the access control file, the user only needs to know that access will be denied.

Hence, the routines must log information about errors. The logged information must enable the system administrator to locate the error in the file. The error message should include the access control file name and line or record number. This suggests that both routines need access to that information. Hence, the record counts, line numbers, and file name must be shared. For reasons of modularity, this implies that these two routines should be in a submodule of the access checking routine. If they are placed in their own module, no other parts of the routine can access the line or record numbers (and none need to, given the design described here). If the module is placed under the access control routine, no external functions can read records from the access control file or check data against that file’s contents.

31.4.4 Summary

This section has examined the development of a program for performing a security-critical function. Beginning with a requirements analysis, the design and parts of the implementation demonstrate the need for repeated analysis to ensure that the design meets the requirements and that design decisions are documented. From the point at which the derivation stopped, the implementation is simple.

We will now discuss some common security-related programming problems. Then we will discuss testing, installation, and maintenance.

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