Allocating and freeing memory in different modules and levels of abstraction burdens the programmer with tracking the lifetime of that block of memory. This may cause confusion regarding when and if a block of memory has been allocated, or freed, leading to programming defects such as double-free vulnerabilities or writing to un-allocated memory.
To avoid these situations, it is recommended that memory be allocated and freed at the same level of abstraction, and preferably in the same code module.
The affects of not following this recommendation are best demonstrated by an actual vulnerability. Freeing memory in different modules resulted in a vulnerability in MIT Kerberos 5 http://web.mit.edu/kerberos/advisories/MITKRB5-SA-2004-002-dblfree.txt. The problem is the MIT Kerberos 5 code contains error-handling logic, which frees memory allocated by the ASN.1 decoders if pointers to the allocated memory are non-null. However, if a detectable error occurs, the ASN.1 decoders themselves free memory which they have allocated. When some library functions receive errors from the ASN.1 decoders, they also attempt to free, causing a double-free vulnerability.
References
- Seacord 05 Chapter 4 Dynamic Memory Management