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Date: Mon, 20 Oct 2014 18:42:19 +0400
From: Andrey Ryabinin <a.ryabinin@samsung.com>
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Subject: Re: drivers: random: Shift out-of-bounds in _mix_pool_bytes
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To: Theodore Ts'o <tytso@mit.edu>
Cc: Daniel Borkmann <dborkman@redhat.com>, Andrew Morton <akpm@linux-foundation.org>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>, Peter Zijlstra <peterz@infradead.org>, Michal Marek <mmarek@suse.cz>, Sasha Levin <sasha.levin@oracle.com>, x86@kernel.org, linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org, Andreas Dilger <adilger.kernel@dilger.ca>, Dmitry Vyukov <dvyukov@google.com>, Konstantin Khlebnikov <koct9i@gmail.com>

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
>