Page History

Integer values used as a size argument to malloc(), calloc(), or realloc() must be valid and large enough to contain the objects to be stored. If size arguments are incorrect or can be manipulated by an attacker, then a buffer overflow may occur. Incorrect size arguments, inadequate range checking, integer overflow, or truncation can result in the allocation of an inadequately sized buffer. The programmer must ensure that size arguments to memory allocation functions allocate sufficient memory.

Non-Compliant Code Example (

...

Integer Overflow)

In this non-compliant code example, cBlocks is multiplied by 16 and the result is stored in the unsigned long long int alloc.

...

If both size_t and unsigned long long types are represented as a 64-bit unsigned value, the result of the multiplication operation may not be representable as an unsigned long long value.

Wiki MarkupSee \[[INT35-C. Evaluate integer expressions in a larger size before comparing or assigning to that size]\] for more information on upcasting.

Compliant Solution (

...

Integer Overflow)

Wiki MarkupIn this compliant solution, the integer values passed as size arguments to memory allocation functions are of the correct size and have not been altered due to integer overflow \[[(INT32-C. Ensure that operations on signed integers do not result in overflow]\] ) or truncation \[[(INT31-C. Ensure that integer conversions do not result in lost or misinterpreted data]\]).

enum { BLOCKSIZE = 16 };
...
void *AllocBlocks(size_t cBlocks) {
  if (cBlocks == 0 || cBlocks > SIZE_MAX / BLOCKSIZE) return NULL;
  return malloc (cBlocks * BLOCKSIZE);
} /* end AllocBlocks */

...

This example checks the value of {{cBlocks}} to make sure the subsequent multiplication operation cannot result in an integer overflow. The code also ensures that {{cBlocks}} is not equal to zero (see \[[MEM04-A. Do not perform zero length allocations]\]).

Non-Compliant Code Example (

...

Range Checking)

In this non-compliant code example, the string referenced by str and the string length represented by len orginate from untrusted sources. The length is used to perform a memcpy() into the fixed size static array buf. The len variable is guaranteed to be less than BUFF_SIZE. However, because len is declared as an int it could have a negative value that would bypass the check. The memcpy() function implicitly converts len to an unsigned size_t type, and the resulting operation results in a buffer overflow.

int len;
char *str;
char buf[BUFF_SIZE];

/* ... */
if (len < BUFF_SIZE){
  memcpy(buf, str, len);
}
/* ... */

Compliant Solution (

...

Range Checking)

In this compliant solution, len is declared as a size_t to there is no possibility of this variable having a negative value and bypassing the range check.

size_t len;
char *str;
char buf[BUFF_SIZE];

/* ... */
if (len < BUFF_SIZE){
  memcpy(buf, str, len);
}
/* ... */

...

See \[[INT01-A. Use rsize_t or size_t for all integer values representing the size of an object]\] for more information on representing the size of objects.

Non-Compliant Code Example (

...

Size Calculation)

In this non-compliant code example, an array of long is allocated and assigned to p. However, sizeof(int) is used to size the allocated memory. If sizeof(long) is larger than sizeof(int) then an insufficient amount of memory is allocated.

...

This example also checks for unsigned integer overflow in compliance with INT32-C. Ensure that operations on signed integers do not result in overflow.

Compliant Solution (

...

Size Calculation)

To correct the non-compliant code example, sizeof(long) is used to size the memory allocation.

...

void function(size_t len) {
   long *p;
   if (len == 0 || len > SIZE_MAX / sizeof(*p)) {
      /* handle overflow */
   }
   p = (long *)malloc(len * sizeof(*p));
   if (p == NULL) {
      /*   handle error */
   }
   /* ... */
   free(p);
}

Wiki MarkupThe code also ensures that {{len}} is not equal to zero (see \[[MEM04-A. Do not perform zero length allocations]\]).

Risk Assessment

Providing invalid size arguments to memory allocation functions can lead to buffer overflows and the execution of arbitrary code with the permissions of the vulnerable process.

...