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Variadic functions provide the ability to specify accept a variable number of arguments to a function, but they can be but are problematic. Variadic functions contain define an implicit contract between the function writer and the function user that must be made to establish how many arguments are passed on an invocation. If care is not exercised when invoking a variadic function to ensure that it knows when to stop processing arguments and that the argument list is used incorrectly, there may be dangerous consequences. It should be noted that variadic functions must always have an ellipsis as a parameter, or the result is undefinedallows the function to determine the number of arguments passed in any particular invocation. Failure to enforce this contract may result in undefined behavior. See undefined behavior 141 of Appendix J of the C Standard.

Argument Processing

In the following code example, a the variadic function called average() (taken from an article written by Robert Seacord for Linux World Magazine on variadic functions) is used to determine calculates the average value of its passed the positive integer arguments passed to the function [Seacord 2013]. The function will stop processing processes arguments when it sees that the argument is -1until it encounters an argument with the value of va_eol (-1).

enum { va_eol = -1 };

unsigned int average(int first, ...) {
  size_tunsigned int count = 0;
  unsigned int sum = 0;
  int i = first;
  va_list markerargs;

  va_start(markerargs, first);

  while (i != -1va_eol) {
    sum += i;
    count++;
    i = va_arg(markerargs, int);
  }

  va_end(markerargs);
  return(count ? (sum / count) : 0);
}

Note that va_start() must always be called to initialize the argument list and that va_end() must always be called when finished with a variable argument list.

Noncompliant Code Example

In this noncompliant code example, the average() However, if the function is called as follows:

int avg = average(1, 4, 6, 4, 1);

The omission of the -1 va_eol terminating value means that on some architectures, the function will continue to grab process values from the stack until it either hits a -1 encounters a va_eol by coincidence , or until it is terminated.

In the following line of code, which is an actual vulnerability in an implementation of a useradd function from the shadow-utils package, the POSIX function open() (which is implemented as a variadic function) is called missing an argument. If the stack is maliciously manipulated, the missing argument, which controls access permissions, could be set to a value that allows for an unauthorized user to read or modify data.

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or an error occurs.

Compliant Solution

This compliant solution enforces the contract by adding va_eol as the final argument:

int avg = average(1, 4, 6, 4, 1, va_eol);

Noncompliant Code Example

Another common mistake is to use more format conversion specifiers than supplied arguments, as shown in this noncompliant code example:

const char *error_msg = "Resource not available to user.";
/* ... */
printf("Error (%s): %s", error_msg);

This code results in nonexistent arguments being processed by the function, potentially leaking information about the process.

Compliant Solution

This compliant solution matches the number of format specifiers with the number of variable arguments This results in undefined behavior, which could end up pulling extra values off the stack and unintentionally exposing data. The following example illustrates a case of this:

const char const *error_msg = "Resource not available to user.";
/* ... */
printf("Error (%s): %s", error_msg);

Argument List Caveats

C99 C functions that themselves take accept the variadic primitive va_list as an argument pose an additional threat when dealing with variadic functionsrisk. Calls to vfprintf(), vfscanf(), vprintf(), vscanf(), vsnprintf(), vsprintf(), and vsscanf() use the va_arg() macro, invalidating the parameterized va_list. ThusConsequently, this once a va_list must not be used except for is passed as an argument to any of these functions, it cannot be used again without a call to the va_end() macro once any of those functions are used followed by a call to va_start().

Risk Assessment

Incorrectly using a variadic function can result in abnormal program termination or unintended information disclosure.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Automated Detection

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Related Guidelines

Bibliography

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References

Wiki Markup\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 7.15, "Variable arguments"; 7.19.6.8 "The {{vfprintf}} function"