MSC15-A. Do not depend on undefined behavior

C99 defines undefined behavior in Section 3.4.3 as:

behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this International Standard imposes no requirements

C99 Section 4 explains how the standard identifies undefined behaviors:

If a "shall" or "shall not" requirement that appears outside of a constraint is violated, the behavior is undefined. Undefined behavior is otherwise indicated in this International Standard by the words "undefined behavior" or by the omission of any explicit definition of behavior. There is no difference in emphasis among these three; they all describe "behavior that is undefined".

C99 Annex J.2, "Undefined behavior" contains a list of explicit undefined behaviors in C99.

Behavior can be classified as undefined by the C standards committee for the following reasons:

• to give the implementor license not to catch certain program errors that are difficult to diagnose.
• to identify areas of possible conforming language extension: the implementor may augment the language by providing a definition of the officially undefined behavior.

Conforming implementations can deal with undefined behavior in a variety of fashions, from ignoring the situation completely with unpredictable results, to translating or executing the program in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic message). Because compilers are not obligated to generate code for undefined behavior, these behaviors are candidates for optimization. By assuming that undefined behaviors will not occur, compilers can generate code with better performance characteristics.

Unfortunately, undefined behaviors do occur, particularly in the presence of an an attacker. Optimizations make it difficult to determine how these systems will behave in the presence of undefined behaviors. This is particularly true when visually inspecting source code which relies on undefined behaviors, a code reviewer cannot be certain if the code will be compiled or if it will be optimized out. Furthermore, just because a compiler currently generates object code for an undefined behavior, does not mean that future versions of the compiler are obligated to do the same; this behavior may be viewed as an opportunity for further optimization. Compilers are also not required to issue diagnostics for undefined behavior, so there is frequently no easy way to identify undefined behavior in code.

All of this puts the onus on the programmer to write strictly conforming code, with or without the help of the compiler. Because performance is a primary emphasis of the C language, this situation is likely to get worse before it gets better.

Non-Compliant Code Example

An example of undefined behavior in C99 is the behavior on signed integer overflow. This non-compliant code example depends on this behavior to catch the overflow:

#include <assert.h>

int foo(int a) {
  assert(a + 100 > a);
  printf("%d %d\n", a + 100, a);
  return a;
}

int main(void) {
  foo(100);
  foo(INT_MAX);
}

This code tests for signed integer overflow by testing to see if a + 100 > a. This test cannot evaluate to false unless an integer overflow occurs. However, because a conforming implementation is not required to generate code for undefined behavior, and signed integer overflow is undefined behavior, this code may be compiled out. For example, gcc version 4.1.1 optimizes out the assertion for all optimization levels and gcc 4.2.3 optimizes out the assertion for program compiled with -O2 level optimization and higher.

On some platforms, the integer overflow will cause the program to terminate (before it has an opportunity to test).

Compliant Solution

This compliant solution does not depend on undefined behavior because it generates code to test for overflow in the assertion.

#include <assert.h>

int foo(int a) {
  assert(a < (INT_MAX - 100));
  printf("%d %d\n", a + 100, a);
  return a;
}

int main(void) {
  foo(100);
  foo(INT_MAX);
}

Risk Assessment

While it is rare that the entire application can be strictly conforming, the goal should be that almost all the code is allowed for a strictly conforming program (which among other things means that it avoids undefined behavior), with the implementation-dependent parts confined to modules that the programmer knows he needs to adapt to the platform when it changes.

Related Vulnerabilities

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

References

[[ISO/IEC 9899:1999]] Section 3.4.3, "undefined behavior," and Section 4, "Conformance," and Annex J.2, "Undefined behavior".
[[ISO/IEC PDTR 24772]] "EWF Undefined Behaviour"
[[Seacord 05]] Chapter 5, "Integers"

MSC14-A. Do not introduce unnecessary platform dependencies      13. Miscellaneous (MSC)       MSC16-A. Limit access to the file system by creating a jail