SEI CERT C Coding Standard

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Dynamic memory management is a common source of programming flaws that can lead to security vulnerabilities. Decisions regarding how dynamic memory is allocated, used, and deallocated are the burden of the programmer. Poor memory management can lead to security issues such as heap-buffer overflows, dangling pointers, and double-free issues \[[Seacord 05a|AA. References#SeacordBibliography#Seacord 05]\]. From the programmer's perspective, memory management involves allocating memory, reading and writing to memory, and deallocating memory.

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To avoid these situations, memory should be allocated and freed at the same level of abstraction, and ideally in the same code module.  This includes the use of the following memory allocation and deallocation functions described in C99 Section 7.20.3 \[[ISO/IEC 9899:1999|AA. References#ISOBibliography#ISO/IEC 9899-1999]\]:

Code Block
void *malloc(size_t size);

void *calloc(size_t nmemb, size_t size);

void *realloc(void *ptr, size_t size);

void free(void *ptr);

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Failing to follow this recommendation has led to real-world vulnerabilities. For example, freeing memory in different modules resulted in a vulnerability in MIT Kerberos 5 \[[MIT 04|AA. References#MITBibliography#MIT 04]\]. The MIT Kerberos 5 code in this case contained error-handling logic, which freed memory allocated by the ASN.1 decoders if pointers to the allocated memory were non-null. However, if a detectable error occurred, the ASN.1 decoders freed the memory that they had allocated. When some library functions received errors from the ASN.1 decoders, they also attempted to free the same memory, resulting in a double-free vulnerability.

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\[[ISO/IEC 9899:1999|AA. References#ISOBibliography#ISO/IEC 9899-1999]\] Section 7.20.3, "Memory Management Functions"
\[[ISO/IEC PDTR 24772|AA. References#ISOBibliography#ISO/IEC PDTR 24772]\] "XYL Memory Leak"
\[[MIT 04|AA. References#MITBibliography#MIT 04]\]
\[[MITRE 07|AA. References#MITREBibliography#MITRE 07]\] [CWE ID 416|http://cwe.mitre.org/data/definitions/416.html], "Use After Free," and [CWE ID 415|http://cwe.mitre.org/data/definitions/415.html], "Double Free"
\[[Plakosh 05|AA. References#PlakoshBibliography#Plakosh 05]\]
\[[Seacord 05a|AA. References#SeacordBibliography#Seacord 05]\] Chapter 4, "Dynamic Memory Management"

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