Re: [PATCH] nullb: Adjust device size calculation in null_alloc_dev()

[Zhu Yanjun <yanjun.zhu@linux.dev>]

在 2024/9/17 15:24, Damien Le Moal 写道:
> On 2024/09/17 16:21, Damien Le Moal wrote:
>> On 2024/09/17 16:07, Aleksandr Mishin wrote:
>>> In null_alloc_dev() device size is a subject to overflow because 'g_gb'
>>> (which is module parameter, may have any value and is not validated
>>> anywhere) is not cast to a larger data type before performing arithmetic.
>>>
>>> Cast 'g_gb' to unsigned long to prevent overflow.
>>>
>>> Found by Linux Verification Center (linuxtesting.org) with SVACE.
>>>
>>> Fixes: 2984c8684f96 ("nullb: factor disk parameters")
>>> Signed-off-by: Aleksandr Mishin <amishin@t-argos.ru>
>>> ---
>>>   drivers/block/null_blk/main.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/block/null_blk/main.c b/drivers/block/null_blk/main.c
>>> index 2f0431e42c49..5edbf9c0aceb 100644
>>> --- a/drivers/block/null_blk/main.c
>>> +++ b/drivers/block/null_blk/main.c
>>> @@ -762,7 +762,7 @@ static struct nullb_device *null_alloc_dev(void)
>>>   		return NULL;
>>>   	}
>>>   
>>> -	dev->size = g_gb * 1024;
>>> +	dev->size = (unsigned long)g_gb * 1024;
>> This still does not prevent overflows... So what about doing a proper check ?
> This still does not prevent overflows on 32-bits architectures.

The max value of "unsigned long" is 2^64 - 1 while the max value of int 
is 2^31 -1.

(2^64 - 1) / (2^31-1) is about 2^33 while 1024 is 2^10.

2^33 is greater than 2^10.

So in the above, it seems that it is difficult to overflow.

If I am missing something, please let me know.

Thanks,

Zhu Yanjun

>
>>>   	dev->completion_nsec = g_completion_nsec;
>>>   	dev->submit_queues = g_submit_queues;
>>>   	dev->prev_submit_queues = g_submit_queues;

-- 
Best Regards,
Yanjun.Zhu