On 16/09/2013 01:56, Daniel Borkmann wrote:
On 09/15/2013 06:59 PM, James Yonan wrote:
On 15/09/2013 09:45, Florian Weimer wrote:
* James Yonan:
+ * Constant-time equality testing of memory regions.
+ * Returns 0 when data is equal, non-zero otherwise.
+ * Fast path if size == 16.
+ */
+noinline unsigned long crypto_mem_not_equal(const void *a, const
void *b, size_t size)
I think this should really return unsigned or int, to reduce the risk
that the upper bytes are truncated because the caller uses an
inappropriate type, resulting in a bogus zero result. Reducing the
value to 0/1 probably doesn't hurt performance too much. It also
doesn't encode any information about the location of the difference in
the result value, which helps if that ever leaks.
The problem with returning 0/1 within the function body of
crypto_mem_not_equal is that it makes it easier for the compiler to
introduce a short-circuit optimization.
It might be better to move the test where the result is compared
against 0 into an inline function:
noinline unsigned long __crypto_mem_not_equal(const void *a, const
void *b, size_t size);
static inline int crypto_mem_not_equal(const void *a, const void *b,
size_t size) {
return __crypto_mem_not_equal(a, b, size) != 0UL ? 1 : 0;
}
This hides the fact that we are only interested in a boolean result
from the compiler when it's compiling crypto_mem_not_equal.c, but also
ensures type safety when users test the return value. It's also
likely to have little or no performance impact.
Well, the code snippet I've provided from NaCl [1] is not really
"fast-path"
as you say, but rather to prevent the compiler from doing such
optimizations
by having a transformation of the "accumulated" bits into 0 and 1 as an end
result (likely to prevent a short circuit), plus it has static size, so no
loops applied here that could screw up.
Variable size could be done under arch/ in asm, and if not available, that
just falls back to normal memcmp that is being transformed into a same
return
value. By that, all other archs could easily migrate afterwards. What do
you
think?
[1] http://www.spinics.net/lists/linux-crypto/msg09558.html
I'm not sure that the differentbits -> 0/-1 transform in NaCl really
buys us anything because we don't care very much about making the final
test of differentbits != 0 constant-time. An attacker already knows
whether the test succeeded or failed -- we care more about making the
failure cases constant-time.
To do this, we need to make sure that the compiler doesn't insert one or
more early instructions to compare differentbits with 0xFF and then
bypass the rest of the F(n) lines because it knows then that the value
of differentbits cannot be changed by subsequent F(n) lines. It seems
that all of the approaches that use |= to build up the differentbits
value suffer from this problem.
My proposed solution is rather than trying to outsmart the compiler with
code that resists optimization, why not just turn optimization off
directly with #pragma GCC optimize. Or better yet, use an optimization
setting that provides reasonable speed without introducing potential
short-circuit optimizations.
By optimizing for size ("Os"), the compiler will need to turn off
optimizations that add code for a possible speed benefit, and the kind
of short-circuit optimizations that we are trying to avoid fall
precisely into this class -- they add an instruction to check if the OR
accumulator has all of its bits set, then if so, do an early exit. So
by using Os, we still benefit from optimizations that increase speed but
don't increase code size.
Once we eliminate the possibility of short-circuit optimizations, then
it's much more straightforward to make the function fast (e.g. by
comparing 64-bits at a time) and flexible (by handling arbitrary size).
My current implementation of crypto_mem_not_equal does both.
James
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