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The type size_t generally covers the entire address space. The C Standard, Annex K (normative), "Bounds-checking interfaces" [ISO/IEC 9899:2011],  introduces ," introduces a new type, rsize_t, defined to be size_t but explicitly used to hold the size of a single object [Meyers 2004]. In code that documents this purpose by using the type rsize_t, the size of an object can be checked to verify that it is no larger than RSIZE_MAX, the maximum size of a normal single object, which provides additional input validation for library functions. See STR07-C. Use the bounds-checking interfaces for remediation of existing string manipulation code for additional discussion of C11 Annex K.

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Signed integer overflow causes undefined behavior. The following are two possible conditions under which this code constitutes a serious vulnerability:

sizeof(size_t) == sizeof(int)

The unsigned n may contain a value greater than INT_MAX. Assuming quiet wraparound on signed overflow, the loop executes n times because the comparison i < n is an unsigned comparison. Once i is incremented beyond INT_MAX, i takes on negative values starting with (INT_MIN). Consequently, the memory locations referenced by p[i] precede the memory referenced by p, and a write outside array bounds occurs.

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In this noncompliant code example, the value of length is read from a network connection and passed as an argument to a wrapper to malloc() to allocate the appropriate data block. Provided that the size of an unsigned long is equal to the size of an unsigned int, and both sizes are equal to or smaller than the size of size_t, this code runs as expected. However, if the size of an unsigned long is greater than the size of an unsigned int, the value stored in length may be truncated when passed as an argument to alloc().  Both read functions return zero on success and nonzero on failure.

void *alloc(unsigned int blocksize) {
  return malloc(blocksize);
}

int read_counted_string(int fd) {
  unsigned long length;
  unsigned char *data;

  if (read_integer_from_network(fd, &length) < 0) {
    return -1;
  }

  data = (unsigned char*)alloc(length+1);
  if (data == NULL) {
    return -1;  /* Indicate failure */
  }

  if (read_network_data(fd, data, length) < 0) {
    free(data);
    return -1;
  }
  data[length-1] = '\0';

  /* ... */
  free( data);
  return 0;
}

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Declaring both length and the blocksize argument to alloc() as rsize_t eliminates the possibility of truncation. This compliant solution assumes that read_integer_from_network() and read_network_data() can also be modified to accept a length argument of type pointer to rsize_t and rsize_t, respectively. If these functions are part of an external library that cannot be updated, care must be taken when casting length into an unsigned long to ensure that integer truncation does not occur.

void *alloc(rsize_t blocksize) {
  if (blocksize == 0 || blocksize > RSIZE_MAX) {
    return NULL;  /* Indicate failure */
  }
  return malloc(blocksize);
}

int read_counted_string(int fd) {
  rsize_t length;
  unsigned char *data;

  if (read_integer_from_network(fd, &length) < 0) {
    return -1;
  }

  data = (unsigned char*)alloc(length+1);
  if (data == NULL) {
    return -1; /* Indicate failure */
  }

  if (read_network_data(fd, data, length) < 0) {
    free(data);
    return -1;
  }
  data[length-1] = '\0';

  /* ... */
  free( data);
  return 0;
}

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The improper calculation or manipulation of an object's size can result in exploitable vulnerabilities.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography

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