linux-kernel - Re: drivers: random: Shift out-of-bounds in _mix_pool

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Date:	Mon, 20 Oct 2014 10:16:35 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Daniel Borkmann <dborkman@...hat.com>
Cc:	Andrey Ryabinin <a.ryabinin@...sung.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Michal Marek <mmarek@...e.cz>,
	Sasha Levin <sasha.levin@...cle.com>, x86@...nel.org,
	linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Konstantin Khlebnikov <koct9i@...il.com>
Subject: Re: drivers: random: Shift out-of-bounds in _mix_pool_bytes

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.  :-)

     	     	   	    	 	     - Ted
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