linux-kernel - Re: scsi: BUG in scsi_init

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Date:   Tue, 31 Jan 2017 07:41:51 -0800
From:   James Bottomley <jejb@...ux.vnet.ibm.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>,
        Johannes Thumshirn <jthumshirn@...e.de>
Cc:     "Martin K. Petersen" <martin.petersen@...cle.com>,
        linux-scsi <linux-scsi@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        syzkaller <syzkaller@...glegroups.com>,
        Hannes Reinecke <hare@...e.de>
Subject: Re: scsi: BUG in scsi_init_io

On Tue, 2017-01-31 at 10:50 +0100, Dmitry Vyukov wrote:
> On Tue, Jan 31, 2017 at 10:20 AM, Johannes Thumshirn <
> jthumshirn@...e.de> wrote:
> > On Tue, Jan 31, 2017 at 09:55:52AM +0100, Dmitry Vyukov wrote:
> > > Hello,
> > > 
> > > The following program triggers BUG in scsi_init_io:
> > 
> > Well crashing a machine just because of an empty dma transfer is a
> > bit harsh,
> > isn't it?
> > 
> > From 86e6fa5f618fe588b98e923e032f33e075fcd4f4 Mon Sep 17 00:00:00
> > 2001
> > From: Johannes Thumshirn <jthumshirn@...e.de>
> > Date: Tue, 31 Jan 2017 10:16:00 +0100
> > Subject: [PATCH] scsi: don't BUG_ON() empty DMA transfers
> > 
> > Don't crash the machine just because of an empty transfer. Use
> > WARN_ON()
> > combined with returning an error.
> > 
> > Signed-off-by: Johannes Thumshirn <jthumshirn@...e.de>
> > ---
> >  drivers/scsi/scsi_lib.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> > index e9e1e14..414588a 100644
> > --- a/drivers/scsi/scsi_lib.c
> > +++ b/drivers/scsi/scsi_lib.c
> > @@ -1040,7 +1040,8 @@ int scsi_init_io(struct scsi_cmnd *cmd)
> >         bool is_mq = (rq->mq_ctx != NULL);
> >         int error;
> > 
> > -       BUG_ON(!blk_rq_nr_phys_segments(rq));
> > +       if (WARN_ON(!blk_rq_nr_phys_segments(rq)))
> > +               return -EINVAL;
> 
> 
> Please-please-please, let's not use WARN for something that is not a
> kernel bug and is user-triggerable.

It is a kernel bug and it should not be user triggerable, so it should
have a warn_on or bug_on.  It means something called a data setup
function with no data.  There's actually a root cause that patches like
this won't fix, can we find it?

James