Re: [BUG/PATCH] kernel RNG and its secrets

[Stephan Mueller <smueller@chronox.de>]

Am Mittwoch, 18. März 2015, 16:09:34 schrieb Hannes Frederic Sowa:

Hi Hannes,

>On Wed, Mar 18, 2015, at 13:42, Daniel Borkmann wrote:
>> On 03/18/2015 01:20 PM, Stephan Mueller wrote:
>> > Am Mittwoch, 18. März 2015, 13:19:07 schrieb Hannes Frederic Sowa:
>> >>>> My proposal would be to add a
>> >>>> 
>> >>>> #define OPTIMIZER_HIDE_MEM(ptr, len) __asm__ __volatile__ ("" :
>> >>>> :
>> >>>> "m"(
>> >>>> ({ struct { u8 b[len]; } *p = (void *)ptr ; *p; }) )
>> >>>> 
>> >>>> and use this in the code function.
>> >>>> 
>> >>>> This is documented in gcc manual 6.43.2.5.
>> >>> 
>> >>> That one adds the zeroization instructuctions. But now there are
>> >>> much
>> >>> more than with the barrier.
>> >>> 
>> >>>    400469:       48 c7 04 24 00 00 00    movq   $0x0,(%rsp)
>> >>>    400470:       00
>> >>>    400471:       48 c7 44 24 08 00 00    movq   $0x0,0x8(%rsp)
>> >>>    400478:       00 00
>> >>>    40047a:       c7 44 24 10 00 00 00    movl   $0x0,0x10(%rsp)
>> >>>    400481:       00
>> >>>    400482:       48 c7 44 24 20 00 00    movq   $0x0,0x20(%rsp)
>> >>>    400489:       00 00
>> >>>    40048b:       48 c7 44 24 28 00 00    movq   $0x0,0x28(%rsp)
>> >>>    400492:       00 00
>> >>>    400494:       c7 44 24 30 00 00 00    movl   $0x0,0x30(%rsp)
>> >>>    40049b:       00
>> >>> 
>> >>> Any ideas?
>> >> 
>> >> Hmm, correct definition of u8?
>> > 
>> > I use unsigned char
>> > 
>> >> Which version of gcc do you use? I can't see any difference if I
>> >> compile your example at -O2.
>> > 
>> > gcc-Version 4.9.2 20150212 (Red Hat 4.9.2-6) (GCC)
>
>Well, was an error on my side, I see the same behavior.
>
>> I can see the same with the gcc version I previously posted. So
>> it clears the 20 bytes from your example (movq, movq, movl) at
>> two locations, presumably buf[] and b[].
>
>Yes, it looks like that. The reservation on the stack changes, too.
>
>Seems like just using barrier() is the best and easiest option.

Would you prepare a patch for that?
>
>Thanks,
>Hannes


Ciao
Stephan