MEM35-C. Ensure that size arguments to memory allocation functions are correct

Integer values used as a size argument to malloc(), calloc(), or realloc() must be valid and large enough to contain the type of object 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 1

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

void* AllocBlocks(size_t cBlocks) {
  if (cBlocks == 0) return NULL;
  unsigned long long alloc = cBlocks * 16;
  return (alloc < UINT_MAX)
     ? malloc(cBlocks * 16)
     : NULL;
}

If size_t is represented as a 32-bit unsigned value and unsigned long long represented as a 64-bit unsigned value, for example, the result of this multiplication can still overflow because the actual multiplication is a 32-bit operation. As a result, the value stored in alloc will always be less than UINT_MAX.

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.

Compliant Solution 1

Make sure that integer values passed as size arguments to memory allocation functions are valid and have not been corrupted due to integer overflow, truncation, or sign error [[Integers (INT)]]. In the following example, the multsize_t() function multiples two values of type size_t and sets errno to a non-zero value if the resulting value cannot be represented as a size_t.

void *AllocBlocks(size_t cBlocks) {
  size_t alloc;

  if (cBlocks == 0) return NULL;
  alloc = multsize_t(cBlocks, 16);
  if (errno) {
    return NULL;
  }
  else {
    return malloc(alloc);
  }
} /* end AllocBlocks */

Non-Compliant Code Example 2

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 2

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);
}
/* ... */

Non-Compliant Code Example 3

In this example, an array of long integers 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 will be allocated. This example also checks for unsigned numeric overflow in compliance with [[INT32-C]].

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

Compliant Solution 3

To correct this example, sizeof(long) is used to size the memory allocation.

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

Alternatively, sizeof(*p) can be used to properly size the allocation:

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

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.

Automated Detection

The Coverity Prevent SIZECHECK checker finds memory allocations that are assigned to a pointer that reference objects larger than the allocated block (Example 3 above). Coverity Prevent cannot discover all violations of this rule so further verification is necessary.

Related Vulnerabilities

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

References

[[ISO/IEC 9899-1999]] Section 7.20.3, "Memory Management Functions"
[[Seacord 05]] Chapter 4, "Dynamic Memory Management," and Chapter 5, "Integer Security"
[Coverity 07] Coverity Prevent User's Manual (3.3.0) (2007).