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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 75 in Annex J of C11 [ISO/IEC 9899:2011].) 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 2006]. The programmer must ensure that size arguments to variable length arrays, especially those derived from untrusted data, are in a valid range.

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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.

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ISO/IEC TR 17961 (Draft) Tainted, potentially mutilated, or out-of-domain integer values are used in a restricted sink [taintsink]

ISO/IEC TR 24772 "XYX Boundary beginning violation" and "XYZ Unchecked array indexing"

Bibliography

[Griffiths 2006]

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