Freeing memory multiple times has similar consequences to accessing memory after it is freed. The underlying data structures that manage the heap can become corrupted in a way that could introduce security vulnerabilities into a program. These types of issues are referred to as double-free vulnerabilities. In practice, double-free vulnerabilities can be exploited to execute arbitrary code. For instance, VU#623332, which describes a double free vulnerability in the MIT Kerberos 5 function krb5_recvauth(). To eliminate double-free vulnerabilities, it is necessary to guarantee that dynamic memory is freed only once. Programmers should be wary when freeing memory in a loop or conditional statement, if coded incorrectly, these constructs can lead to double-free vulnerabilities.
Non-compliant Code Example 1
In this example, the memory referred to by x may freed twice: once if error_condition is true and again at the end of the code.
x = malloc (number * sizeof(int));
if (x == NULL) {
/* Handle Allocation Error */
}
/* ... */
if (error_conditon == 1) {
/* Handle Error Condition*/
free(x);
}
/* ... */
free(x);
Compliant Solution 1
Only free a pointer to dynamic memory referred to by x once. This is accomplished by removing the call to free() in the section of code executed when error_condition is true.
x = malloc (number * sizeof(int));
if (x == NULL) {
/* Handle Allocation Error */
}
/* ... */
if (error_conditon == 1) {
/* Handle Error Condition*/
}
/* ... */
free(x);
References
- VU#623332 http://www.kb.cert.org/vuls/id/623332
- MIT krb5 Security Advisory 2005-003 http://web.mit.edu/kerberos/www/advisories/MITKRB5-SA-2005-003-recvauth.txt
- OWASP Double Free http://www.owasp.org/index.php/Double_Free