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

[Andrey Ryabinin]

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));
}


> 	input_rotate = (input_rotate + (i ? 7 : 14)) & 31;
> 
> Just to be sure I've tried adding a:
> 
> 	WARN_ON(input_rotate >= 32);
> 
> before the rol32 line, and it's not triggering for me after booting
> under kvm using an i386 kernel.
> 
> Is this something you can reliably reproduce?  Can you try putting a
> WARN_ON before the rol32() on a kernel w/o usbsan, just to make sure
> this isn't some kind of false positive?  And then can you tell me
> something more about the .config you are using to build your test kernel?
> 
> Thanks,
> 
> 						- Ted
> 

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