Local, automatic variables can assume unexpected values if they are used before they are initialized. C99 specifies "If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate." [[ISO/IEC 9899-1999]] In practice, this value defaults to whichever values are currently stored in stack memory. While unitialized memory often contains zero, this is not guaranteed. This can consequently cause a program to behave in an unpredictable or unplanned manner, and may provide an avenue for attack. Some compilers warn about unitialized variables, but these can be ignored by the programmer. As a result, it is necessary to guarantee that all local variables are initialized with a default value. The value assigned should be documented as the default value for that variable in the comments associated with that variable's declaration.
Non-Compliant Code Example 1
In this example, two functions are called consecutively. The first function, func1(...) is passed an integer entered by a user. That integer is stored in variable: i for the duration of the function. The second function func2() declares a local integer variable: j. j is not initialized before being checked against a constant value, CONDITION_CHECK. Because j is uninitialized, it assumes whatever value is at that location in the stack, in this case the value of i from func1(). As a result, if the user entered 42, the condition statement if (j == CONDITION_CHECK) succeeds.
#define CONDITION_CHECK 42
void func1(int arg) { int i = arg; }
void func2(void) {
int j;
if (j == CONDITION_CHECK) puts("Condition passed!!\n");
else puts("ERROR: Condition failed\n");
}
...
func1(i); /* the value of i originates from an untrusted source */
func2();
...
Compliant Solution 1
The local, automatic variable j should be initialized to a default value.
#define CONDITION_CHECK 42
void func1(int arg) { int i = arg; }
void func2(void) {
int j = 0; /* initialize j to 0 */
if (j == CONDITION_CHECK) puts("Condition passed!!\n");
else puts("ERROR: Condition failed\n");
}
...
func1(i); /* the value of i originates from an untrusted source */
func2();
...
Non-compliant Code Example 2
In this example, user input is copied to the character array buf. The logit() function copies the user input to another buffer, buffer, and prints it to standard output. Next, the runit() routine is called. This routine declares an array of 50 characters, buf, and a character pointer, ptr. However, because ptr is not initialized it references data used by the last function called, in this case, the contents of the user controlled data copied in the logit() function. When the data referred to by ptr is copied to buf using an unbounded strcpy(), ptr is dereferenced and the data in that location is copied to buf. If the memory referred to by ptr contains more than 50 characters without a null character, a buffer overflow will occur.
#define BUF_SIZE 150
void runit(void) {
char buf[50];
char *ptr;
memset(buf,0,50);
strcpy(buf,ptr);
}
void logit(char *str) {
char buffer[BUF_SIZE];
strcpy(buffer, str);
printf("The message: %s\nhas been logged\n",buffer);
}
int main(int argc, char *argv[]) {
char buf[BUF_SIZE];
strcpy(buf, argv[1]);
logit(buf);
runit();
}
Compliant Solution 2
The local variable ptr should be initialized to a default value, in this case NULL.
#define BUF_SIZE 150
void runit(void) {
char buf[50];
char *ptr = NULL; /* initialize ptr to 0 */
memset(buf,0,50);
strcpy(buf,ptr);
}
void logit(char *str) {
char buffer[BUF_SIZE];
int i;
for (i=0; i < BUF_SIZE; ++i) buffer[i] = '\0';
strcpy(buffer, str);
printf("The message: %s\nhas been logged\n",buffer);
}
int main(int argc, char *argv[]) {
char buf[BUF_SIZE];
strcpy(buf, argv[1]);
logit(buf);
runit();
}
References
- ISO/IEC 9899-1999 Section 6.7.8 Initialization