CTR33-CPP. Guarantee that copies are made into storage of sufficient size

Copying data into an array that is not large enough to hold that data results in a buffer overflow. To prevent such errors, data copied to the destination array must be restricted based on the size of the destination array, or, preferably, the destination array must be guaranteed to be large enough to hold the data to be copied.

Vulnerabilities that result from copying data to an undersized buffer often involve null-terminated character arrays (NTCA). Consult VOID STR31-CPP. Guarantee that storage for character arrays has sufficient space for character data and the null terminator for specific examples of this rule that involve NTCA.

Most such copies are made with the memcpy() function. However the memmove() function and the memset() function can also have the same vulnerabilities because they overwrite a block of memory without checking that the block is valid.

Noncompliant Code Example (Array)

Improper use of functions that limit copies with a size specifier, such as memcpy(), may result in a buffer overflow. In this noncompliant code example, an array of integers is copied from src to dest using memcpy(). However, the programmer mistakenly specified the amount to copy based on the size of src, which is stored in len, rather than the space available in dest. If len is greater than 256, then a buffer overflow will occur.

enum { WORKSPACE_SIZE = 256 };

void func(const int src[], size_t len) {
  int dest[WORKSPACE_SIZE];
  memcpy(dest, src, len * sizeof(int));
  /* ... */
}

Compliant Solution (Array)

The amount of data copied should be limited based on the available space in the destination buffer. This can be accomplished by adding a check to ensure the amount of data to be copied from src can fit in dest.

enum { WORKSPACE_SIZE = 256 };

void func(const int src[], size_t len) {
  int dest[WORKSPACE_SIZE];
  if (len > WORKSPACE_SIZE) {
      /* Handle Error */
  }
  memcpy(dest, src, sizeof(int)*len);
  /* ... */
}

Noncompliant Code Example (Vector)

Vectors can be subject to the same vulnerabilities. The copy function provides no inherent bounds checking, and can lead to a buffer overflow. In this noncompliant code example, a vector of integers is copied from src to dest using copy(). Since copy() does nothing to expand the dest vector, thus the program will overflow the buffer on copying the first element.

#include <algorithm>

void func(const vector<int> src) {
  vector<int> dest;
  copy( src.begin(), src.end(), dest.begin());
  /* ... */
}

This hazard applies to any algorithm that takes a 'destination' iterator, expecting to fill it with values. Most of the STL algorithms expect the destination container to have sufficient space to hold the values provided.

Compliant Solution (Vector, resize())

The proper way to use copy() is to ensure the destination container can hold all the elements being copied to it. This code example enlarges the capacity of the vector before starting the copyo.

#include <algorithm>

void func(const vector<int> src) {
  vector<int> dest;
  dest.resize( src.size());
  copy( src.begin(), src.end(), dest.begin());
  /* ... */
}

Compliant Solution (Vector, back_inserter())

An alternative safe approach is to supply a back_insert_iterator as the destination argument. This iterator expands the destination container by one element for each element supplied by the algorithm. This guarantees the destination container will become sufficiently large enough to hold the elements provided.

#include <algorithm>

void func(const vector<int> src) {
  vector<int> dest;
  copy( src.begin(), src.end(), back_inserter( dest));
  /* ... */
}

The front_insert_iterator works in a similar fashion to the back_insert_iterator, but it automatically pushes new elements to the front of the container, causing them to be listed in the container in reverse order. Since it uses the member push_front() method, it is not available for vectors.

Also note that using insert iterators is less efficient than using resize() becuase they expand the destination container one element at a time.

Risk Assessment

Copying data to a buffer that is too small to hold that data results in a buffer overflow. Attackers can exploit this condition to execute arbitrary code.

Automated Detection

Coverity Code Advisor version 7.5 can detect violations of this rule.

Fortify SCA Version 5.0 can detect violations of this rule.

Splint Version 3.1.1 can detect violations of this rule.

Compass/ROSE can detect some violations of this rule.

Klocwork Version 8.0.4.16 can detect violations of this rule with the ABR checker.

Related Vulnerabilities

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

Other Languages

This rule appears in the C Secure Coding Standard as void ARR33-C. Guarantee that copies are made into storage of sufficient size.

Bibliography

[ISO/IEC PDTR 24772] "XYB Buffer Overflow in Heap," "XYW Buffer Overflow in Stack," and "XYZ Unchecked Array Indexing"
[Meyers 01] Item 30: Make sure destination ranges are big enough
[MITRE] CWE ID 119, "Failure to Constrain Operations within the Bounds of an Allocated Memory Buffer"
[MITRE] CWE ID 805, "Buffer Access with Incorrect Length Value"
[Seacord 05a] Chapter 2, "Strings"
[VU#196240]

CTR32-CPP. Use valid references, pointers, and iterators to reference elements of a container      06. Containers (CTR)      CTR34-CPP. Use Valid Iterator Ranges