use-after-free / double-free exploit mitigation

[Martin Sebor]

On 09/07/2017 10:00 AM, Florian Weimer wrote:
> On 09/06/2017 02:46 PM, up201407890@alunos.dcc.fc.up.pt wrote:
>> What are your thoughts on adding a SAFE_FREE() macro to glibc:
>>
>> #define SAFE_FREE(x) do { if((x) != 0x0) { free(x); (x) = (void *)0x1; }
>> } while(0)
>>
>> After free(x), we set x to an address that will crash when dereferenced
>> (use-after-free), and will also crash when it's an argument to free().
>> Note that NULL isn't used, because free(NULL) does nothing, which might
>> hide potential double-free bugs.
>
> Maybe GCC should optionally do this for the actual call to free.  There
> is some debate to what extend pointer *values* remain valid after free.
> Martin Sebor may have some thought on that.
>
> In any case, some GCC assistance is needed so that
>
>   free (some_struct->ptr);
>   free (some_struct);
>
> actually clobbers some_struct->ptr.  I don't think we want to call out
> to explicit_bzero here.

One of the advantages of doing this in the compiler (besides not
having to change source code) is distinguishing rvalues from lvalues.

Martin