use-after-free / double-free exploit mitigation

[Federico Manuel Bento]

A 2017-09-10 16:41, Martin Sebor escreveu:
> On 09/09/2017 11:59 AM, Federico Manuel Bento 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
>> 
>> Perhaps this sould be used when making use of FORTIFY_SOURCE?
> 
> That seems reasonable.  David Malcolm has done some preliminary work
> on a GCC maaloc/free optimization and diagnostic pass that might be
> well suited to this sort of instrumentation.  Opening an enhancement
> request in GCC Bugzilla for this would help track interest in
> the feature.
> 
> Martin

Here's the request in GCC's Bugzilla:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82179

Thanks,
Federico Bento.