On 4/15/14 10:33 AM, stefan.n...@t-online.de wrote:
Hi,
I have "checked" the current source code of 'crpyto/mem.c' and I'm a
little bit suprised that no memset()-calls are made before the free_*()
functions are entered. I think a "zeroing" of the previous used memory
is a good solutions to beware for accessing old memory content.
Leaving aside the problem that just zeroing the memory simply
doesn't work (for a start into that discussion see e.g.
http://bytes.com/topic/c/answers/660296-memset-free), there is
OPENSSL_cleanse which does something similar (actually, it
overwrites the memory with "garbage", not just with zeros) in a
way that works. Attempting to be faster at run time, this needs to be
called explicitly, though (and it's called in a lot of places if you look
into the source code).
But it might in fact be a good idea to put that call simply in the
free function and be done with it. With modern processors, the
slowdown is probably hardly nocticeable anyway.
Regards,
Stefan
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OpenSSL Project http://www.openssl.org
Development Mailing List openssl-dev@openssl.org
Automated List Manager majord...@openssl.org
Here is a means of using memset so that it can't be optimized out.
#include <stdint.h>
#include <string.h>
void *
safe_memset(void *s, int c, size_t n)
{
if (n > 0) {
volatile unsigned volatile_zero = 0;
volatile uint8_t *vs = (volatile uint8_t*)s;
do {
memset(s, c, n);
} while (vs[volatile_zero] != (uint8_t)c);
}
return s;
}
Since vs points to a volatile, the load in the while clause actually has
to be done. That forces the compiler to actually store c into at least
the byte that is tested, in practice byte zero. But the fact that the
index is volatile zero, and since it is volatile it could spontaneously
change to anything, the compiler has to store c into all bytes.
The key observation is that while you can't pass a volatile to memset
(you get a warning and the volatile gets stripped away), you can use a
volatile in a test that could go the wrong way if the memset were elided.
Could you C language lawyers please check this out and make sure I've
not made a mistake.
Thank you,
--David
______________________________________________________________________
OpenSSL Project http://www.openssl.org
Development Mailing List openssl-dev@openssl.org
Automated List Manager majord...@openssl.org
