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

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Date:	Mon, 20 Oct 2014 10:13:21 -0400
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Daniel Borkmann <dborkman@...hat.com>,
	Andrey Ryabinin <a.ryabinin@...sung.com>
CC:	"Theodore Ts'o" <tytso@....edu>,
	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>, 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 10:09 AM, Daniel Borkmann wrote:
> On 10/20/2014 03:58 PM, Andrey Ryabinin wrote:
>> On 10/20/2014 04:49 PM, Theodore Ts'o wrote:
>>> On Mon, Oct 20, 2014 at 03:03:22PM +0400, Andrey Ryabinin wrote:
>>>> Hi, Theodore.
>>>>
>>>> I've got this while booting kernel with ubsan:
>>>>
>>>>      [    0.000000] ================================================================================
>>>>      [    0.000000] UBSan: Undefined behaviour in ../include/linux/bitops.h:107:33
>>>>      [    0.000000] shift exponent 32 is to large for 32-bit type 'unsigned int'
>>>        ...
>>>>      [    0.000000] _mix_pool_bytes (/home/andrew/linux/ubsan_x86//include/linux/bitopsh:107 /home/andrew/linux/ubsan_x86//drivers/char/randomc:509)
>>>
>>> So this doesn't make any sense to me.   This is triggering here:
>>>
>>>     w = rol32(*bytes++, input_rotate);
>>>
>>> .... but input_rotate should never be >= 32, since it is set this way:
>>>
>>
>> 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.

It's an issue because it's _undefined_. For all you know gcc could return
whatever it wants as the result (42?) - no one promises you a "0" there.


Thanks,
Sasha
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