use-after-free / double-free exploit mitigation

[Federico Manuel Bento]

>>> 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

Perhaps this sould be used when making use of FORTIFY_SOURCE?

Federico.