Re: More on in-memory zeroisation

[Thierry Moreau]

Jack:

Thank you for pointing this. I must admit you point to an inescapable 
counter-example for my analysis.

Maybe global optimization was not a significant factor in the 1980's 
when the C standard language was established -- it does refer to 
external linkage and "genuine function".

In the case of volatile declaration, the GCC 4.2.2 compiler gave me a 
warning that the volatile qualifier was ignored because the memset 
formal parameter declaration does not match. At least, as a compiler 
user I get a proper warning message.

Regards

 - Thierry Moreau


Original message:

Jack Lloyd wrote:

On Wed, Dec 12, 2007 at 05:27:38PM -0500, Thierry Moreau wrote:

As a consequence of alleged consensus above, my understanding of the C 
standard would prevail and (memset)(?,0,?) would refer to an external 
linkage function, which would guarantee (to the sterngth of the above 
consensus) resetting an arbitrary memory area for secret intermediate 
result protection.


GCC on x86-64 (-O2) compiles this function to the same machine code
regardless of the value of ZEROIZE:

#include <string.h>

int sensitive(int key)
   {
   char buf[16];
   int result = 0;
   size_t j;

   for(j = 0; j != sizeof(buf); j++)
      buf[j] = key + j;

   for(j = 0; j != sizeof(buf); j++)
      result += buf[j];

#if ZEROIZE
   (memset)(buf, 0, sizeof(buf));
#endif

   return result;
   }

Even if (memset) must refer to a function with external linkage (an
analysis I find dubious), there is nothing stopping the compiler from
doing IPA/whole program optimization - especially with a very basic
function like memset (in the code above, if buf is declared volatile,
GCC does do the memset: but it does it by moving immediate zero values
directly to the memory locations, not by actually jumping to any
external function).

Regards,
  Jack

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