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Subject: Re: [PATCH] usb: storage: sddr55: Fix a possible null-pointer
 dereference in sddr55_transport()
From:   Oliver Neukum <oneukum@suse.com>
To:     Jia-Ju Bai <baijiaju1990@gmail.com>, gregkh@linuxfoundation.org,
        stern@rowland.harvard.edu
Cc:     usb-storage@lists.one-eyed-alien.net, linux-kernel@vger.kernel.org,
        linux-usb@vger.kernel.org
Date:   Mon, 29 Jul 2019 13:15:22 +0200
In-Reply-To: <20190729100555.2081-1-baijiaju1990@gmail.com>
References: <20190729100555.2081-1-baijiaju1990@gmail.com>
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Am Montag, den 29.07.2019, 18:05 +0800 schrieb Jia-Ju Bai:

Hi,

> In sddr55_transport(), there is an if statement on line 836 to check
> whether info->lba_to_pba is NULL:
>     if (info->lba_to_pba == NULL || ...)
> 
> When info->lba_to_pba is NULL, it is used on line 948:
>     pba = info->lba_to_pba[lba];
> 
> Thus, a possible null-pointer dereference may occur.

Yes, in practice READ_CAPACITY will always be called and set
up the correct translation table, but you can probably exploit
this.

> To fix this bug, info->lba_to_pba is checked before being used.
> 
> This bug is found by a static analysis tool STCheck written by us.
> 
> Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
> ---
>  drivers/usb/storage/sddr55.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/storage/sddr55.c b/drivers/usb/storage/sddr55.c
> index b8527c55335b..50afc39aa21d 100644
> --- a/drivers/usb/storage/sddr55.c
> +++ b/drivers/usb/storage/sddr55.c
> @@ -945,7 +945,8 @@ static int sddr55_transport(struct scsi_cmnd *srb, struct us_data *us)
>  			return USB_STOR_TRANSPORT_FAILED;
>  		}
>  
> -		pba = info->lba_to_pba[lba];
> +		if (info->lba_to_pba)
> +			pba = info->lba_to_pba[lba];

If you use that fix, pba will be uninitialized when used. It should be
something like:

pba = info->lba_to_pba ? info->lba_to_pba[lba] : 0;

	Regards
		Oliver