Re: [PATCH] Use memzero_explicit to clear local buffers

[Giel van Schijndel]

On Mon, Jan 05, 2015 at 08:35:38 +1100, Herbert Xu wrote:
> On Sun, Jan 04, 2015 at 07:05:40PM +0100, Giel van Schijndel wrote:
>> When leaving a function use memzero_explicit instead of memset(0) to
>> clear locally allocated/owned buffers. memset(0) may be optimized away.
>> 
>> All of the affected buffers contain sensitive data, key material or
>> derivatives of one of those two.
> 
> Nack.

Do you mean that the sample below doesn't contain sensitive data?
Or is there another buffer(s) in my patch that you believe doesn't
contain that?

(I contain a hash derived from secret material to be a "derivative of one of
those two", leaking of which could lead to a confirmation-attack).

>> diff --git a/arch/x86/crypto/sha256_ssse3_glue.c b/arch/x86/crypto/sha256_ssse3_glue.c
>> index 8fad72f..b616e63 100644
>> --- a/arch/x86/crypto/sha256_ssse3_glue.c
>> +++ b/arch/x86/crypto/sha256_ssse3_glue.c
>> @@ -164,7 +164,7 @@ static int sha256_ssse3_final(struct shash_desc *desc, u8 *out)
>>  		dst[i] = cpu_to_be32(sctx->state[i]);
>>  
>>  	/* Wipe context */
>> -	memset(sctx, 0, sizeof(*sctx));
>> +	memzero_explicit(sctx, sizeof(*sctx));
> 
> sctx does not point to stack memory so this is bogus.
> 
> Only stack memory cleared just before it goes out of scope needs
> memzero_explicit.

Is that because the compiler can't safely optimize memset(0) away for a
variable with greater-than-local scope?

Because I think using memzero_explicit() as an indicator that said
buffer contains data that really *needs* to be destroyed is enough of a
reason already. I believe any overhead is negligable because there's
only a single extra call involved and that's cheap for the extra clarity
it buys (i.e. "this piece of memory *really* needs to be destroyed
beyond this statement"). (Though this approach is only valid for memory
that can contain security-sensitive data IMO.)

-- 
Met vriendelijke groet,
With kind regards,
Giel van Schijndel
--
"Always code as if the guy who ends up maintaining your code will be a
 violent psychopath who knows where you live."
  -- Rick Osborne

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