Potential double free¶
ID: cpp/double-freeKind: path-problemSecurity severity: 9.3Severity: warningPrecision: highTags: - reliability - security - external/cwe/cwe-415Query suites: - cpp-code-scanning.qls - cpp-security-extended.qls - cpp-security-and-quality.qls
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Deallocating memory more than once can lead to a double-free vulnerability. This can be exploited to corrupt the allocator’s internal data structures, which can lead to denial-of-service attacks by crashing the program, or security vulnerabilities, by allowing an attacker to overwrite arbitrary memory locations.
Recommendation¶
Ensure that all execution paths deallocate the allocated memory at most once. In complex cases it may help to reassign a pointer to a null value after deallocating it. This will prevent double-free vulnerabilities since most deallocation functions will perform a null-pointer check before attempting to deallocate memory.
Example¶
In the following example,buff is allocated and then freed twice:
int*f(){int*buff=malloc(SIZE*sizeof(int));do_stuff(buff);free(buff);int*new_buffer=malloc(SIZE*sizeof(int));free(buff);// BAD: If new_buffer is assigned the same address as buff,// the memory allocator will free the new buffer memory region,// leading to use-after-free problems and memory corruption.returnnew_buffer;}
Reviewing the code above, the issue can be fixed by simply deleting the additional call tofree(buff).
int*f(){int*buff=malloc(SIZE*sizeof(int));do_stuff(buff);free(buff);// GOOD: buff is only freed once.int*new_buffer=malloc(SIZE*sizeof(int));returnnew_buffer;}
In the next example,task may be deleted twice, if an exception occurs inside thetry block after the firstdelete:
voidg(){MyTask*task=nullptr;try{task=newMyTask;...deletetask;...}catch(...){deletetask;// BAD: potential double-free}}
The problem can be solved by assigning a null value to the pointer after the firstdelete, as callingdelete a second time on the null pointer is harmless.
voidg(){MyTask*task=nullptr;try{task=newMyTask;...deletetask;task=nullptr;...}catch(...){deletetask;// GOOD: harmless if task is NULL}}
References¶
OWASP:Doubly freeing memory.
Common Weakness Enumeration:CWE-415.