linux-kernel - Re: [PATCHv2] Check whether divisor is non-zero before division

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Date: Thu, 23 May 2024 09:58:21 -0400
From: Alan Stern <stern@...land.harvard.edu>
To: Shichao Lai <shichaorai@...il.com>
Cc: gregkh@...uxfoundation.org, oneukum@...e.com, linux-usb@...r.kernel.org,
  usb-storage@...ts.one-eyed-alien.net, linux-kernel@...r.kernel.org,
  xingwei lee <xrivendell7@...il.com>, yue sun <samsun1006219@...il.com>
Subject: Re: [PATCHv2] Check whether divisor is non-zero before division

On Thu, May 23, 2024 at 05:26:08PM +0800, Shichao Lai wrote:
> Since uzonesize may be zero, so judgements for non-zero are nessesary in both place.
> Previous check is moved out of loop, and one more check is added in alauda_write_lba.
> 
> Reported-by: xingwei lee <xrivendell7@...il.com>
> Reported-by: yue sun <samsun1006219@...il.com>
> Signed-off-by: Shichao Lai <shichaorai@...il.com>
> ---
>  drivers/usb/storage/alauda.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/usb/storage/alauda.c b/drivers/usb/storage/alauda.c
> index 115f05a6201a..a6e60ef5cb0d 100644
> --- a/drivers/usb/storage/alauda.c
> +++ b/drivers/usb/storage/alauda.c
> @@ -818,6 +818,8 @@ static int alauda_write_lba(struct us_data *us, u16 lba,
>  	unsigned int blocksize = MEDIA_INFO(us).blocksize;
>  	unsigned int lba_offset = lba % uzonesize;
>  	unsigned int new_pba_offset;
> +	if (!uzonesize)
> +		return USB_STOR_TRANSPORT_ERROR;
>  	unsigned int zone = lba / uzonesize;
>  
>  	alauda_ensure_map_for_zone(us, zone);
> @@ -923,6 +925,8 @@ static int alauda_read_data(struct us_data *us, unsigned long address,
>  	unsigned int uzonesize = MEDIA_INFO(us).uzonesize;
>  	struct scatterlist *sg;
>  	int result;
> +	if (!uzonesize)
> +		return USB_STOR_TRANSPORT_ERROR;
>  
>  	/*
>  	 * Since we only read in one block at a time, we have to create

This is definitely NOT the right way to fix the bug!

uzonesize is set once, when the device is probed, in 
alauda_init_media().  That is where the check belongs; if uzonesize is 0 
then the function should print a warning and return 
USB_STOR_TRANSPORT_ERROR, because the device is unusable.

It's probably a good idea to check pagesize, blocksize, and zonesize at 
the same time, even though none of them are used for any divisions.

Alan Stern