linux-kernel - Re: scsi: BUG in scsi_init

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Message-ID: <CACT4Y+Y0Yi43kN+sAOd=pu4FTnW4wPeGwiDHEU45s6DTXs_JmQ@mail.gmail.com>
Date:   Tue, 31 Jan 2017 10:50:49 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Johannes Thumshirn <jthumshirn@...e.de>
Cc:     jejb@...ux.vnet.ibm.com,
        "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, 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. This makes it impossible to
automate kernel testing and requires hiring an army of people doing
mechanical job of sorting out WARNING reports into kernel-bugs and
non-kernel-bugs.
If the message is absolutely necessary (while kernel does not
generally explain every EINVAL on console), the following will do:

      if (!blk_rq_nr_phys_segments(rq)) {
              pr_err("you are doing something wrong\n");
              return -EINVAL;
      }


>         error = scsi_init_sgtable(rq, &cmd->sdb);
>         if (error)
> --
> 2.10.2
>
>
> --
> Johannes Thumshirn                                          Storage
> jthumshirn@...e.de                                +49 911 74053 689
> SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
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