linux-kernel - Re: [PATCH] usb: storage: return on error to avoid a null pointer dereference

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Date:   Thu, 6 Jul 2017 10:45:10 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Colin King <colin.king@...onical.com>
cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        <linux-usb@...r.kernel.org>,
        <usb-storage@...ts.one-eyed-alien.net>,
        <kernel-janitors@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usb: storage: return on error to avoid a null pointer
 dereference

On Thu, 6 Jul 2017, Colin King wrote:

> From: Colin Ian King <colin.king@...onical.com>
> 
> When us->extra is null the driver is not initialized, however, a
> later call to osd200_scsi_to_ata is made that dereferences
> us->extra, causing a null pointer dereference.  The code
> currently detects and reports that the driver is not initialized;
> add a return to avoid the subsequent dereference issue in this
> check.
> 
> Detected by CoverityScan, CID#100308 ("Dereference after null check")
> 
> Signed-off-by: Colin Ian King <colin.king@...onical.com>
> ---
>  drivers/usb/storage/isd200.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/storage/isd200.c b/drivers/usb/storage/isd200.c
> index fba4005dd737..174ad60679e7 100644
> --- a/drivers/usb/storage/isd200.c
> +++ b/drivers/usb/storage/isd200.c
> @@ -1529,8 +1529,10 @@ static void isd200_ata_command(struct scsi_cmnd *srb, struct us_data *us)
>  
>  	/* Make sure driver was initialized */
>  
> -	if (us->extra == NULL)
> +	if (us->extra == NULL) {
>  		usb_stor_dbg(us, "ERROR Driver not initialized\n");
> +		return;
> +	}

Good catch, but before returning you need to set

	srb->result = DID_ERROR << 16;

so that the SCSI layer will realize the command wasn't handled.

Alan Stern

>  
>  	scsi_set_resid(srb, 0);
>  	/* scsi_bufflen might change in protocol translation to ata */
>