Variable-length arrays (VLA) are essentially the same as traditional C arrays, except that they are declared with a size that is not a constant integer expression, and may be declared only at block scope or function prototype scope and no linkage. A variable-length array can be declared as follows:

{   /* block scope */
    char vla[size];
}

where the integer expression {{size}} and the declaration of {{vla}} are both evaluated at runtime. If the size argument supplied to a variable-length array is not a positive integer value, the behavior is undefined (see [undefined behavior 69|CC. Undefined Behavior#ub_69] in Annex J of C99). In addition, if the magnitude of the argument is excessive the program may behave in an unexpected way. An attacker may be able to leverage this behavior to overwrite critical program data \[[Griffiths 06|AA. Bibliography#Griffiths 06]\]. The programmer must ensure that size arguments to variable-length arrays are valid and have not been corrupted as the result of an exceptional integer condition.

Noncompliant Code Example

In this noncompliant code example, a variable-length array of size size is declared. The size is declared as size_t in compliance with INT01-C. Use rsize_t or size_t for all integer values representing the size of an object.

void func(size_t size) {
  int vla[size];
  /* ... */
}
/* ... */

However, it is not guaranteed that the value of size is a valid size argument, potentially giving rise to a security vulnerability.

Compliant Code Solution

This compliant solution ensures the size argument used to allocate vla is in a valid range (between 1 and a programmer-defined maximum), otherwise it uses an algorithm that relies on dynamic memory allocation.

enum { MAX_ARRAY = 1024 };

void func(size_t size) {
  if (0 < size && size < MAX_ARRAY) {
    int vla[size];
    /* ... */
  } else {
    /* Use dynamic allocation */
  }
}
/* ... */

Implementation Details

Microsoft

Variable length arrays are not supported by Microsoft compilers.

GCC

Newer versions of GCC have incorporated variable length arrays, but do not yet claim full C99 conformance. As such, variable length arrays should only be used on GCC with great care.

On an example Debian GNU/Linux Intel 32-bit test machine with GCC v. 4.2.2, the value of a variable length array's size is interpreted as a 32-bit signed integer. Passing in a negative number for the size will likely cause the program stack to become corrupted, and passing in a large positive number may cause a terminal stack overflow. It is important to note that this information may become outdated as GCC evolves.

Risk Assessment

Failure to properly specify the size of a variable length array may allow arbitrary code execution or result in stack exhaustion.

Automated Detection

The Coverity Prevent Version 5.0 REVERSE_NEGATIVE, NEGATIVE_RETURNS checker can find the instances where data is read/write from a negative array index.

Related Vulnerabilities

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

References

\[[Griffiths 06|AA. Bibliography#Griffiths 06]\]
\[[ISO/IEC PDTR 24772|AA. Bibliography#ISO/IEC PDTR 24772]\] "XYX Boundary Beginning Violation" and "XYZ Unchecked Array Indexing"

ARR31-C. Use consistent array notation across all source files      06. Arrays (ARR)      ARR33-C. Guarantee that copies are made into storage of sufficient size