RE: [PATCH] scsi: zero per-cmd driver data for each MQ I/O

[Long Li]

> -----Original Message-----
> From: Bart Van Assche [mailto:Bart.VanAssche@xxxxxxxxxxx]
> Sent: Monday, May 15, 2017 4:03 PM
> To: jejb@xxxxxxxxxxxxxxxxxx; linux-scsi@xxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; Long Li <longli@xxxxxxxxxxxxx>;
> martin.petersen@xxxxxxxxxx
> Cc: Long Li <longli@xxxxxxxxxxxxx>
> 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@xxxxxxxxxxxxx>
> >
> > 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@xxxxxxxxxxxxx>
> > ---
> >  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 for looking! Yes this is for chasing a bug. 

Actually scsi_mq_prep_fn() doesn't touch cmd->req (which is the struct scsi_request mentioned in your email).

With the patch, we also zero the private data used by lower layer driver, in addition to the private data in scsi_cmnd.

> 
> Thanks,
> 
> Bart.