ARR38-C. Guarantee that library functions do not form invalid pointers (Draft Stage)

Definitions

According to the WG14 Document [1]:

Given an integer expression E, the derived type T of E is determined as follows:

• if E is a sizeof expression, then T is the type of the operand of the expression;

• otherwise, if E is an integer identifier, then T is the derived type of the expression last used to store a value in E;

• otherwise, if the derived type of each of E's subexpressions is the same, then T is that type;

• otherwise, the derived type is an unspecified character type compatible with any of char, signed char, and unsigned char.

Example:      int val;

                     int arr [ARR_SIZE];

                     size_t c1 = sizeof (val);

                     size_t c2 =sizeof (arr) / sizeof (val);

                     size_t c3 = sizeof (arr) / sizeof (*arr);

The derived type in this example is int for c1 and c2 (because both subexpressions have the same type, int) and is an unspecified character type compatible with any of char, signed char, and unsigned char for c3.

The effective size of a pointer is the size of the object to which it points.

                    int arr[5];

                    int *p = arr;

The effective size of the pointer p in this example is sizeof(arr)---that is, 5*sizeof(int).

The effective type of an object is defined as either its declared type or (if its type isn't declared) the effective type of the value assigned to it.

                   char *p;

The effective type of pointer p in this case is char.

                   void *p;

                   p = obj;

In this case, pointer p's type is not declared, but it is later assigned obj. The effective type of p is therefore equal to the effective type of obj.

Rule Description

C library functions that make changes to arrays or objects usually take at least two arguments: a pointer to the array or object and an integer indicating the number of elements or bytes to be manipulated. If the arguments are supplied improperly during such a function call, the function may cause the pointer to not point to the object at all or to point past the end of the object, leading to undefined behavior. 

To make sure this does not happen, programmers must keep in mind the following rules when using such functions:

• For func (p,n), where p is a pointer, n is an integer, and func is a library function, the value of n should not be greater than the effective size of the pointer. In situations where n is an expression (see the second noncompliant and compliant examples that follow), the effective type of the pointer should be compatible with either the derived type of n or unsigned char.
• For func (p,q, n), where p and q are both pointers, n is an integer, and func is a library function, the value of n should not be greater than the effective size of any of the two pointers (p and q). The effective type of p should be compatible with the derived type of n or unsigned char when n is an expression. Similarly, the effective type of p should be compatible with the effective type of q or unsigned char.
• For expression E of the form T* q = func (n), where func is a memory allocation function, the value of n should not be less than sizeof(T). Also, the effective type of T should be compatible with either the derived type of n or unsigned char.

Library Functions to Which the Rules Can Apply

*Both functions take more than one size_t argument. In such cases, the compliant code must be changed according to the purpose of these arguments. For example, in the case of fread():

size_t fread ( void *ptr, size_t size, size_t count, FILE * stream)

the programmer should make sure the memory block to which ptr points is of at least (size*count) bytes.

Noncompliant Code Example

This noncompliant code example assigns a value greater than the size of dynamic memory to n, which is then passed to memset().

void f1 (size_t nchars) {

	char *p = (char *)malloc(nchars);
	const size_t n = nchars + 1;

	memset(p, 0, n);
	/* More program code */

}

Compliant Solution

This compliant solution ensures that the value of n is not greater than the size of the dynamic memory pointed to by the pointer p:

void f1 (size_t nchars, size_t val) {

	char *p = (char *)malloc(nchars);
	const size_t n = val;

	if (nchars < n) {
     		/* Handle Error */
	}

	else {
		memset(p, 0, n);
	}

}

Noncompliant Code Example

In this noncompliant code example, the effective type of *p is float, and the derived type of the expression n is int. This is calculated using the first rule from the WG14 Document's [1] definition of derived types (see Definitions section). Because n here is a sizeof expression, its derived type is equal to the type of the operand, which is int.

void f2() {


	const size_t ARR_SIZE = 4;
	float a[ARR_SIZE];
	const size_t n= sizeof(int) * ARR_SIZE;
	void *p = a;


	memset(p, 0, n);
	/* More program code */

}

Note: This code could possibly be safe on architectures where sizeof(int) is equal to sizeof(float).

Compliant Solution

In this compliant solution, the derived type of n is also float.

void f2() {

	const size_t ARR_SIZE = 4;
	float a[ARR_SIZE];
	const size_t n = sizeof(float) * ARR_SIZE;
	void *p = a;

	memset(p, 0, n);
	/* More program code */

}

Noncompliant Code Example

In this noncompliant code example, the size of n could be greater than the size of *p. Also, the effective type of *p (int) is not same as the effective type of *q (float).

void f3(int *a) {

	float b = 3.14;
	const size_t n = sizeof(*b);
	void *p = a;
	void *q = &b;

	memcpy(p, q, n);
	/* More program code */

}

Note: This code could possibly be safe on architectures where sizeof(int) is equal to sizeof(float).

Compliant Solution

This compliant solution ensures that the value of n is not greater than the minimum of the effective sizes of *p and *q and that the effective types of the two pointers are also same (float).

void f3(float *a, size_t val) {

	float b = 3.14;
	const size_t n = val;
	void *p = a;
	void *q = &b;

	if( (n > sizeof(a)) || (n > sizeof(b)) ) {
		/* Handle error */
	}
	else {
		memcpy(p, q, n);
		/* More program code */
	}

}

Noncompliant Code Example

In this noncompliant code example, the value of n is greater than the size of T---that is, sizeof(wchar_t). But the derived type of expression n (wchar_t *) is not same as the type of T because its derived type (see Definitions section) will be equal to the type of p, which is wchar_t *. The derived type of n is calculated using the first rule from the WG14 Document's [1] definition of derived types (see Definitions section). Because n here is a sizeof expression, its derived type is equal to the type of the operand (p), which is wchar_t *.

wchar_t *f7() {

	const wchar_t *p = L"Hello, World!";
	const size_t n = sizeof(p) * (wcslen(p) + 1);

	wchar_t *q = (wchar_t *)malloc(n);
	return q;

}

Compliant Solution

This compliant solution ensures that the derived type of n (wchar_t) is same as the type of T (wchar_t) and that the value of n is not less than the size of T.

wchar_t *f7() {

	const wchar_t *p = L"Hello, World!";
	const size_t n = sizeof(wchar_t) * (wcslen(p) + 1);

	wchar_t *q = (wchar_t *)malloc(n);
	return q;

}

Risk Assessment

Depending on the library function called, the attacker may be able to use a heap overflow vulnerability to run arbitrary code. The detection of checks specified in description can be automated, but the remediation has to be manual.

Related Guidelines

API00-C. Functions should validate their parameters (https://www.securecoding.cert.org/confluence/display/seccode/API00-C.+Functions+should+validate+their+parameters)

WG14 Document: N1579, Rule 5.34 Forming invalid pointers by library functions

Bibliography

[1] WG14 Document: N1579, Section 4.5