linux-kernel - Re: [PATCH] lib: memcmp_nta: add timing-attack secure memcmp

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Date:	Mon, 11 Feb 2013 23:58:06 +0100
From:	Daniel Borkmann <borkmann@...earbox.net>
To:	Florian Weimer <fw@...eb.enyo.de>
CC:	gregkh@...uxfoundation.org, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] lib: memcmp_nta: add timing-attack secure memcmp

On 02/11/2013 08:00 PM, Florian Weimer wrote:
 > * Daniel Borkmann:

Thanks for your feedback, Florian!

 >> + * memcmp_nta - memcmp that is secure against timing attacks
 >
 > It's not providing an ordering, so it should not have "cmp" in the
 > name.

I agree. What would you suggest? Probably, it would make sense to
integrate this into the Linux crypto API and name it sth like ...

   crypto_mem_verify(const void *,const void *,__kernel_size_t)

... which returns:

   == 0 - mem regions equal each other
   != 0 - mem regions do not equal each other

 >> +	for (su1 = cs, su2 = ct; 0 < count; ++su1, ++su2, count--)
 >> +		res |= (*su1 ^ *su2);
 >
 > The compiler could still short-circuit this loop.  Unlikely at
 > present, but this looks like a maintenance hazard.

So then better we leave out '|' as a possible candidate and rewrite it as:

+	for (su1 = cs, su2 = ct; 0 < count; ++su1, ++su2, count--)
+		res += (*su1 ^ *su2);
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