linux-kernel - Re: [PATCH v2] usb: storage: sddr55: Fix a possible null-pointer dereference in sddr55

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Date:   Mon, 29 Jul 2019 09:44:21 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Jia-Ju Bai <baijiaju1990@...il.com>
cc:     gregkh@...uxfoundation.org, USB list <linux-usb@...r.kernel.org>,
        USB Storage list <usb-storage@...ts.one-eyed-alien.net>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] usb: storage: sddr55: Fix a possible null-pointer
 dereference in sddr55_transport()

On Mon, 29 Jul 2019, Jia-Ju Bai wrote:

> 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.
> 
> 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.

This is not the right way to fix the bug.

The code already contains a test on line 938:

		if (lba >= info->max_log_blks) {

If this test fails, the driver doesn't try to dereference 
info->lba_to_pba.

The problem is that info->max_log_blks may be set even though 
info->lba_to_pba is NULL, because the READ_CAPACITY case in 
sddr55_transport() doesn't check the return code from 
sddr55_read_map().  _That_ is what needs to be fixed.

Alan Stern

> Signed-off-by: Jia-Ju Bai <baijiaju1990@...il.com>
> ---
> v2:
> * Avoid uninitialized access of pba.
>   Thank Oliver for helpful advice.
> 
> ---
>  drivers/usb/storage/sddr55.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/storage/sddr55.c b/drivers/usb/storage/sddr55.c
> index b8527c55335b..d23aff16091e 100644
> --- a/drivers/usb/storage/sddr55.c
> +++ b/drivers/usb/storage/sddr55.c
> @@ -945,7 +945,7 @@ static int sddr55_transport(struct scsi_cmnd *srb, struct us_data *us)
>  			return USB_STOR_TRANSPORT_FAILED;
>  		}
>  
> -		pba = info->lba_to_pba[lba];
> +		pba = info->lba_to_pba ? info->lba_to_pba[lba] : 0;
>  
>  		if (srb->cmnd[0] == WRITE_10) {
>  			usb_stor_dbg(us, "WRITE_10: write block %04X (LBA %04X) page %01X pages %d\n",
>