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As noted in undefined behavior 169 of Annex J of [ISO/IEC 9899-1999], the behavior a program is undefined when

the pointer argument to the free or realloc function does not match a pointer earlier returned by calloc, malloc, or realloc, or the space has been deallocated by a call to free or realloc.

Freeing memory multiple times has similar consequences to accessing memory after it is freed. (See rule MEM30-C. Do not access freed memory.) First, reading a pointer to deallocated memory is undefined because the pointer value is indeterminate and can have a trap representation. In the latter case, doing so can cause a hardware trap. When reading a freed pointer doesn't cause a trap, the underlying data structures that manage the heap can become corrupted in a way that can 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. One example of this is VU#623332, which describes a double-free vulnerability in the MIT Kerberos 5 function krb5_recvauth().

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Note that this solution checks for numeric overflow. (See rule INT32-C. Ensure that operations on signed integers do not result in overflow.)

Noncompliant Code Example (realloc())

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According to the C99 standard [ISO/IEC 9899-1999] (7.20.3):

If the size of the space requested is zero, the behavior is implementation defined: either a null pointer is returned, or the behavior is as if the size were some nonzero value, except that the returned pointer shall not be used to access an object.

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CERT C Secure Coding Standard: MEM04-C. Do not perform zero length allocations

ISO/IEC TR 24772 "XYK Dangling Reference to Heap" and "XYL Memory Leak"

MITRE CWE: CWE-415, "Double Free"

Bibliography

[MIT 2005]
[OWASP, Double Free]
[Viega 2005] "Doubly freeing memory"
[VU#623332]

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