linux-kernel - Re: [Possible Phish Fraud][PATCH] scsi: zero per-cmd driver data for each MQ I/O

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Date:   Mon, 15 May 2017 23:02:56 +0000
From:   Bart Van Assche <Bart.VanAssche@...disk.com>
To:     "jejb@...ux.vnet.ibm.com" <jejb@...ux.vnet.ibm.com>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "longli@...hange.microsoft.com" <longli@...hange.microsoft.com>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>
CC:     "longli@...rosoft.com" <longli@...rosoft.com>
Subject: Re: [Possible Phish Fraud][PATCH] scsi: zero per-cmd driver data for
 each MQ I/O

On Wed, 2017-05-10 at 14:07 -0700, Long Li wrote:
> From: Long Li <longli@...rosoft.com>
> 
> Lower layer driver may not initialize private data before use. Zero them
> out to prevent use of stale data.
> 
> Signed-off-by: Long Li <longli@...rosoft.com>
> ---
>  drivers/scsi/scsi_lib.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> index 19125d7..a821593 100644
> --- a/drivers/scsi/scsi_lib.c
> +++ b/drivers/scsi/scsi_lib.c
> @@ -1850,7 +1850,7 @@ static int scsi_mq_prep_fn(struct request *req)
>  
>  	/* zero out the cmd, except for the embedded scsi_request */
>  	memset((char *)cmd + sizeof(cmd->req), 0,
> -		sizeof(*cmd) - sizeof(cmd->req));
> +		sizeof(*cmd) - sizeof(cmd->req) + shost->hostt->cmd_size);
>  
>  	req->special = cmd;

Hello Long,

Sorry but this patch looks wrong to me. Since scsi_mq_prep_fn() is called
after scsi_req_init(), erasing struct scsi_request from scsi_mq_prep_fn()
will erase the values that were set by scsi_req_init(). That includes
information like the pointer to the SCSI CDB and the CDB itself. See e.g.
scsi_execute().

Did you come up with this patch after source reading or did you come up
with this patch while chasing a bug?

Thanks,

Bart.