Re: drivers: random: Shift out-of-bounds in _mix_pool_bytes

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Oct 20, 2014 at 10:16:35AM -0400, Theodore Ts'o wrote:
> On Mon, Oct 20, 2014 at 04:09:30PM +0200, Daniel Borkmann wrote:
> > >
> > >It's triggering when input_rotate == 0, so UBSan complains about right shift in rol32()
> > >
> > >static inline __u32 rol32(__u32 word, unsigned int shift)
> > >{
> > >	return (word << shift) | (word >> (32 - shift));
> > >}
> > 
> > So that would be the case when the entropy store's input_rotate calls
> > _mix_pool_bytes() for the very first time ... I don't think it's an
> > issue though.
> 
> I'm sure it's not an issue, but it's still true that 
> 
> 	return (word << 0) | (word >> 32);
> 
> is technically not undefined, and while it would be unfortunate (and
> highly unlikely) if gcc were to say, start nethack, it's technically
> allowed by the C spec.  :-)

In fact, n >> 32 == n.

#include <stdio.h>

int main(int argc, char **argv)
{
	int i = atoi(argv[1]);
	int shift = atoi(argv[2]);
	printf("%x\n", i >> shift);
	return 0;
}

$ ./shift 5 32
5

On x86 at least the shift ops simply mask out the upper bits and
therefore the 32 == 0.

So you end up OR-ing the same value twice, which is harmless.

So no misbehaviour on the rol32() function.

I think I've ran into this before, in that case I did get fail because I
did indeed expect the 0 and things didn't work out.