MEM35-C. Allocate sufficient memory for an object

Integer values used as a size argument to malloc(), calloc(), realloc(), or aligned_alloc() must be of sufficient size 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.

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. Use the correct type of the object when computing the size of chunk 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 (Integer)

In this noncompliant code example, an array of long is allocated and assigned to p. The code checks for unsigned integer overflow in compliance with INT32-C. Ensure that operations on signed integers do not result in overflow and also ensures that len is not equal to zero (see MEM04-C. Beware of zero-length allocations.) Unfortunately, because sizeof(int) is used to size the allocated memory an insufficient amount of memory can be allocated on implementations where sizeof(long) is larger than sizeof(int) .

#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 <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)

#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);
}

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 <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->tm_sec = sec;
  tmb->tm_min = min;
  tmb->tm_hour = hour;
  tmb->tm_mday = day;
  tmb->tm_mon = mon;
  tmb->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.

#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->tm_sec = sec;
  tmb->tm_min = min;
  tmb->tm_hour = hour;
  tmb->tm_mday = day;
  tmb->tm_mon = mon;
  tmb->tm_year = year;
  return tmb;
}

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

Bibliography