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

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 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.

• The first rule from the above definition is applied to non-compliant code/compliant solution 2 and 4 in this page to calculate the derived type of expression 'n' is those examples.

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

     Example:                                     int arr[5];

                                                        int *p = arr;

The effective size of the pointer 'p' in this example will be sizeof(arr) i.e. 5*sizeof(int).

Effective type of an object is defined as either its declared type or (in case its type hasn't been declared) the effective type of the value assigned to it. In the examples below, we have used terms like 'effective type of pointer p' which implies that if the type of 'p' has been declared (eg: char *p) then that type (in this case char) is the effective type of the pointer. If the type is not declared (eg: void *p) and then the pointer is assigned a value (p = obj), then the effective type of 'p' is the effective type of 'obj'.

C library functions that make changes to arrays or objects usually take at least two arguments: i.)  a pointer to the array/object ii.) 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 point past the end of the object. This would lead to undefined behavior. 

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

• For func (p,n), where 'p' is the pointer, 'n' is the integer and 'func' is the library function, the value of â€˜n’ should not be greater than the effective size of the pointer. Also, in situations where 'n' is an expression (see <a href="#cs2">non-compliant code/compliant solution 2</a> below) 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 the integer and 'func' is the library function, the value of â€˜n’ should not be greater than the effective size of any of the two pointers ('p' and 'q'). The effective type of the 'p' should be compatible with the derived type of 'n' or unsigned char. Similarly, the effective type of the '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.

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 makes sure 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 the noncompliant code example below, the effective type of *p is float while the derived type of the expression 'n' 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: A possibility of this code being safe would be on architectures where sizeof (int) is equal to sizeof (float).

<a name="cs2">Compliant Solution</a>

In this compliant solution, the derived type of 'n' is also float (since it is a sizeof expression and therefore the derived is equal to the type of the operand, which is float; see derived type above)

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: A possibility of this code being safe would be on architectures where sizeof (int) is equal to sizeof (float).

Compliant Solution

This compliant solution makes sure that the value of 'n' is not greater the the minimum of effective sizes of *p and *q and the effective types of the two pointers is 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' i.e. sizeof (wchar_t). But, the derived type of expression 'n' (wchar_t *) is not same as the type of 'T' because its derived type (from the definition above; see derived type) will be equal to the type of '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 makes sure that the derived type of 'n' (wchar_t) is same as the type of 'T' (wchar_t). Also, 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;

}

Given below is a non-exhaustive list of library functions to which the above rules can apply:

* - both the functions take more than one size_t argument. In such cases, the compliant code will have to 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 that the memory block to which 'ptr' points is of at least (size*count) bytes.

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