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Date:	Mon, 20 Oct 2014 18:42:19 +0400
From:	Andrey Ryabinin <a.ryabinin@...sung.com>
To:	Theodore Ts'o <tytso@....edu>
Cc:	Daniel Borkmann <dborkman@...hat.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 10/20/2014 06:16 PM, 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.  :-)
> 

There are plenty of such technically not defined but really
harmless things in kernel.
I've reported only few UBs which seemed to me like a real bugs.
So if it's not an issue we could ignore it, OTOH it would be nice
to fix it to shut up ubsan.

>      	     	   	    	 	     - Ted
> 

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