MEM35-C. Allocate sufficient memory for an object

The types of integer expressions used as size arguments to malloc(), calloc(), realloc(), or aligned_alloc() must have sufficient range to represent the size of 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.

Typically, the amount of memory to allocate will be the size of the type of object to allocate. When allocating space for an array, the size of the object will be multiplied by the bounds of the array. When allocating space for a structure containing a flexible array member, the size of the array member must be added to the size of the structure. (See MEM33-C. Allocate and copy structures containing a flexible array member dynamically.) Use the correct type of the object when computing the size of memory to allocate.

STR31-C. Guarantee that storage for strings has sufficient space for character data and the null terminator is a specific instance of this rule.

Noncompliant Code Example (Pointer)

In this noncompliant code example, inadequate space is allocated for a struct tm object because the size of the pointer is being used to determine the size of the pointed-to object:

#include <stdlib.h>
#include <time.h>
 
struct tm *make_tm(int year, int mon, int day, int hour,
                   int min, int sec) {
  struct tm *tmb;
  tmb = (struct tm *)malloc(sizeof(tmb));
  if (tmb == NULL) {
    return NULL;
  }
  *tmb = (struct tm) {
    .tm_sec = sec, .tm_min = min, .tm_hour = hour,
    .tm_mday = day, .tm_mon = mon, .tm_year = year
  };
  return tmb;
}

Compliant Solution (Pointer)

 In this compliant solution, the correct amount of memory is allocated for the struct tm object. When allocating  space for a single object, passing the (dereferenced) pointer type to the sizeof operator is a simple way to allocate sufficient memory. Because the sizeof operator does not evaluate its operand, dereferencing an uninitialized or null pointer in this context is well-defined behavior.

#include <stdlib.h>
#include <time.h>
 
struct tm *make_tm(int year, int mon, int day, int hour,
                   int min, int sec) {
  struct tm *tmb;
  tmb = (struct tm *)malloc(sizeof(*tmb));
  if (tmb == NULL) {
    return NULL;
  }
  *tmb = (struct tm) {
    .tm_sec = sec, .tm_min = min, .tm_hour = hour,
    .tm_mday = day, .tm_mon = mon, .tm_year = year
  };
  return tmb;
}

Noncompliant Code Example (Integer)

In this noncompliant code example, an array of long is allocated and assigned to p. The code attempts to check for unsigned integer overflow in compliance with INT30-C. Ensure that unsigned integer operations do not wrap and also ensures that len is not equal to zero. (See MEM04-C. Beware of zero-length allocations.) However, because sizeof(int) is used to compute the size, and not sizeof(long), an insufficient amount of memory can be allocated on implementations where sizeof(long) is larger than sizeof(int), and filling the array can cause a heap buffer overflow.

#include <stdint.h>
#include <stdlib.h>
 
void function(size_t len) {
  long *p;
  if (len == 0 || len > SIZE_MAX / sizeof(long)) {
    /* Handle overflow */
  }
  p = (long *)malloc(len * sizeof(int));
  if (p == NULL) {
    /* Handle error */
  }
  free(p);
}

Compliant Solution (Integer)

This compliant solution uses sizeof(long) to correctly size the memory allocation:

#include <stdint.h>
#include <stdlib.h>

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

Compliant Solution (Integer)

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

#include <stdint.h>
#include <stdlib.h>
 
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);
}

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

if (a < SIZE_MAX / b && a > 0) ...

Related Vulnerabilities

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

Related Guidelines

Key here (explains table format and definitions)

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-680 and MEM35-C

Intersection( INT32-C, MEM35-C) = Ø

CWE-680 = Union( MEM35-C, list) where list =

• Overflowed buffers with inadequate sizes not produced by integer overflow

CWE-467 and MEM35-C

CWE-467 = Subset( MEM35-C)

CWE-789 and MEM35-C

Intersection( MEM35-C, CWE-789) =

• Insufficient memory allocation on the heap

MEM35-C – CWE-789 =

• Insufficient memory allocation with trusted value but incorrect calculation

CWE-789 - MEM35-C =

• Sufficient memory allocation (possibly over-allocation) with untrusted value

CWE-120 and MEM35-C

Intersection( MEM35-C, CWE-120) = Ø

CWE-120 specifically addresses buffer overflow operations, which occur in the context of string-copying. MEM35-C specifically addresses allocation of memory ranges (some of which may be for subsequent string copy operations).

Consequently, they address different sections of code, although one (or both) may be responsible for a single buffer overflow vulnerability.

CWE-131 and MEM35-C

• Intersection( INT30-C, MEM35-C) = Ø

• CWE-131 = Union( MEM35-C, list) where list =

• Miscalculating a buffer for a non-heap region (such as a variable-length array)

Bibliography