Variable length arrays (VLA) are essentially the same as traditional C arrays, the major difference being they are declared with a size that is not a constant integer expression. A variable length array can be declared as follows:

char vla[s];

The above statement is evaluated at runtime, allocating storage for {{s}} characters in stack memory. If a size argument supplied to VLAs is not a positive integer value of reasonable size, then the program may behave in an unexpected way. An attacker may be able to leverage this behavior to overwrite critical program data \[[Griffiths 06|http://felinemenace.org/papers/p63-0x0e_Shifting_the_Stack_Pointer.txt]\]. The programmer must ensure that size arguments to VLAs are valid and have not been corrupted as the result of an exceptional integer condition.

Non-Compliant Code Example

In this example, a VLA of size s is declared. In accordance with recommendation INT01-A. Use rsize_t or size_t for all integer values representing the size of an object, s is of type size_t, as it is used to specify the size of an object. However, it is unclear whether the value of s is a valid size argument. Depending on how VLAs are implemented, s may be interpreted as a negative value or a very large positive value. In either case, this may result in a security vulnerability.

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

Compliant Code Solution

Validate size arguments used in VLA declarations. The solution below ensures the size argument, s, used to allocate vla is in a valid range: 1 to a user defined constant.

enum { MAX_ARRAY = 1024 };

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

/* ... */
if (s < MAX_ARRAY && s != 0) {
   func(s);
}
else {
   /* Handle Error */
}
/* ... */

Implementation Details

Variable length arrays are not supported by Microsoft compilers.

Risk Assessment

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

Related Vulnerabilities

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

References

\[[Griffiths 06|AA. C References#Griffiths 06]\]

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