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

[Zhu Yanjun]

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@xxxxxxxxxx>
> > > ---
> > >   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;

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Best Regards,
Yanjun.Zhu