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

[Hannes Frederic Sowa <hannes@stressinduktion.org>]

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.

Thanks,
Hannes