Re: [PATCH] crypto: more robust crypto_memneq

[Daniel Borkmann <dborkman@redhat.com>]

On 11/25/2013 04:59 PM, James Yonan wrote:
> On 24/11/2013 14:12, Cesar Eduardo Barros wrote:
>> Disabling compiler optimizations can be fragile, since a new
>> optimization could be added to -O0 or -Os that breaks the assumptions
>> the code is making.
>>
>> Instead of disabling compiler optimizations, use a dummy inline assembly
>> (based on RELOC_HIDE) to block the problematic kinds of optimization,
>> while still allowing other optimizations to be applied to the code.
>>
>> The dummy inline assembly is added after every OR, and has the
>> accumulator variable as its input and output. The compiler is forced to
>> assume that the dummy inline assembly could both depend on the
>> accumulator variable and change the accumulator variable, so it is
>> forced to compute the value correctly before the inline assembly, and
>> cannot assume anything about its value after the inline assembly.
>>
>> This change should be enough to make crypto_memneq work correctly (with
>> data-independent timing) even if it is inlined at its call sites. That
>> can be done later in a followup patch.
>>
>> Compile-tested on x86_64.
>>
>> Signed-off-by: Cesar Eduardo Barros <cesarb@cesarb.eti.br>
>
> This approach using __asm__ ("" : "=r" (var) : "0" (var)) to try to prevent compiler optimizations of var is interesting.
>
> I like the fact that it's finer-grained than -Os and doesn't preclude inlining.

Agreed. This looks much better than the Makefile workaround. Do we have
a hard guarantee that in future, this will not be detected and optimized
away by the compiler?

Otherwise, works fine, e.g.:
int main(void)
{
         int foo = 5;
         __asm__ __volatile__ ("" : "=r" (foo) : "0" (foo));
         if (foo == 5)
                 return 1;
         else
                 return 0;
}

gcc -O2 -Wall foo.c, w/ asm code:
Dump of assembler code for function main:
    0x0000000000400390 <+0>:	mov    $0x5,%eax
    0x0000000000400395 <+5>:	cmp    $0x5,%eax
    0x0000000000400398 <+8>:	sete   %al
    0x000000000040039b <+11>:	movzbl %al,%eax
    0x000000000040039e <+14>:	retq

gcc -O2 -Wall foo.c, w/o asm code:
Dump of assembler code for function main:
    0x0000000000400390 <+0>:	mov    $0x1,%eax
    0x0000000000400395 <+5>:	retq

> One concern would be that __asm__ could be optimized out unless __volatile__ is present.
>
> James