API01-C: Avoid laying out strings in memory directly before sensitive data

Strings (both character and wide-character) are often subject to buffer overflows, which will overwrite the memory immedately past the string. Many rules warn against buffer overflows, including STR31-C. Guarantee that storage for strings has sufficient space for character data and the null terminator and STR35-C. Do not copy data from an unbounded source to a fixed-length array. Sometimes the danger of buffer overflows can be minimized by ensuring that arranging memory such that data that might be corrupted by a buffer overflow is not sensitive.

Noncompliant Code Example

This noncompliant code example stores a set of strings using a linked list.

const size_t String_Size = 20;
struct node_s {
  char name[String_Size];
  struct node_s* next;
}

A buffer overflow on name would overwrite the next pointer, which could then be used to read or write to arbitrary memory.

Compliant Solution

This compliant solution creates a linked list of strings, but stores the next pointer before the string.

const size_t String_Size = 20;
struct node_s {
  struct node_s* next;
  char name[String_Size];
}

If buffer overflow occurs on name, the next pointer remains uncorrupted.

Compliant Solution

In this compliant solution, the linked list stores pointers to strings which are stored elsewhere. Storing the strings elsewhere serves to protect the next pointer from buffer overflows on the strings.

const size_t String_Size = 20;
struct node_s {
  struct node_s* next;
  char* name;
}

Exceptions

API01:EX0: Using a string before sensitive data such as pointers is permitted when it is not practical to segregate the strings from the sensitive data.

These two code examples each creates a linked list of strings, but each node is actually stored inside an array. This serves to ensure that the string is always in front of a next pointer, regardless of how they are ordered in the struct.

const size_t String_Size = 20;
struct node_s {
  char name[String_Size];
  struct node_s* next;
}
struct node_s list[10];

const size_t String_Size = 20;
struct node_s {
  struct node_s* next;
  char name[String_Size];
}
struct node_s list[10];