Subject: Re: [PATCH 36/64] scsi: ibmvscsi: Avoid multi-field memset() overflow by aiming at srp

From: Kees Cook <[email-masked]>
Date: Wed, 28 Jul 2021 11:57:15 -0700

On Tue, Jul 27, 2021 at 09:39:39PM -0400, Martin K. Petersen wrote:
> 
> Kees,
> 
> > In preparation for FORTIFY_SOURCE performing compile-time and run-time
> > field bounds checking for memset(), avoid intentionally writing across
> > neighboring fields.
> >
> > Instead of writing beyond the end of evt_struct->iu.srp.cmd, target the
> > upper union (evt_struct->iu.srp) instead, as that's what is being wiped.
> >
> > Signed-off-by: Kees Cook <[email-masked]>
> 
> Orthogonal to your change, it wasn't immediately obvious to me that
> SRP_MAX_IU_LEN was the correct length to use for an srp_cmd. However, I
> traversed the nested unions and it does look OK.

Yeah, I had the same fun. Maybe I should add a BUILD_BUG_ON() here to
help illustrate the relationship? I did that in a few other places where
the equalities weren't very clear.

For example, change it to:

+	BUILD_BUG_ON(sizeof(evt_struct->iu.srp) != SRP_MAX_IU_LEN);
+	memset(&evt_struct->iu.srp, 0x00, sizeof(evt_struct->iu.srp));
 	srp_cmd = &evt_struct->iu.srp.cmd;
-	memset(srp_cmd, 0x00, SRP_MAX_IU_LEN);

> 
> For good measure I copied Tyrel and Brian.
> 
> Acked-by: Martin K. Petersen <[email-masked]>

For the moment, I'll leave the patch as-is unless you prefer having
the BUILD_BUG_ON(). :)

Thanks!

-Kees

> 
> > ---
> >  drivers/scsi/ibmvscsi/ibmvscsi.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/scsi/ibmvscsi/ibmvscsi.c b/drivers/scsi/ibmvscsi/ibmvscsi.c
> > index e6a3eaaa57d9..7e8beb42d2d3 100644
> > --- a/drivers/scsi/ibmvscsi/ibmvscsi.c
> > +++ b/drivers/scsi/ibmvscsi/ibmvscsi.c
> > @@ -1055,8 +1055,8 @@ static int ibmvscsi_queuecommand_lck(struct scsi_cmnd *cmnd,
> >  		return SCSI_MLQUEUE_HOST_BUSY;
> >  
> >  	/* Set up the actual SRP IU */
> > +	memset(&evt_struct->iu.srp, 0x00, SRP_MAX_IU_LEN);
> >  	srp_cmd = &evt_struct->iu.srp.cmd;
> > -	memset(srp_cmd, 0x00, SRP_MAX_IU_LEN);
> >  	srp_cmd->opcode = SRP_CMD;
> >  	memcpy(srp_cmd->cdb, cmnd->cmnd, sizeof(srp_cmd->cdb));
> >  	int_to_scsilun(lun, &srp_cmd->lun);
> 
> -- 
> Martin K. Petersen	Oracle Linux Engineering

-- 
Kees Cook