Re: use-after-free / double-free exploit mitigation

[Federico Manuel Bento <up201407890 at fc dot up dot pt>]

> > > 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.