Re: Linux 3.0 oopses when pulling a USB CDROM

Dan Williams
2011-07-18T16:59:46Z

On Sat, Jul 2, 2011 at 12:59 PM, Alan Stern <stern@rowland.harvard.edu> wrote:
> On Sat, 2 Jul 2011, Andi Kleen wrote:
>
>> > The problem is that blk_peek_request() calls scsi_prep_fn(), which
>> > does this:
>> >
>> >     struct scsi_device *sdev = q->queuedata;
>> >     int ret = BLKPREP_KILL;
>> >
>> >     if (req->cmd_type == REQ_TYPE_BLOCK_PC)
>> >             ret = scsi_setup_blk_pc_cmnd(sdev, req);
>> >     return scsi_prep_return(q, req, ret);
>> >
>> > It doesn't check to see if sdev is NULL, nor does
>> > scsi_setup_blk_pc_cmnd().  That accounts for this error:
>>
>> I actually added a NULL check in scsi_setup_blk_pc_cmnd early on,
>> but that just caused RCU CPU stalls afterwards and then eventually
>> a hung system.
>
> The RCU problem is likely to be a separate issue.  It might even be a
> result of the use-after-free problem with the elevator.
>
> At any rate, it's clear that the crash in the refcounting log you
> posted occurred because scsi_setup_blk_pc_cmnd() called
> scsi_prep_state_check(), which tried to dereference the NULL pointer.
>
> Would you like to try this patch to see if it fixes the problem?  As I
> said before, I'm not certain it's the best thing to do, but it worked
> on my system.
>
> Alan Stern
>
>
>
>
> Index: usb-3.0/drivers/scsi/scsi_lib.c
> ===================================================================
> --- usb-3.0.orig/drivers/scsi/scsi_lib.c
> +++ usb-3.0/drivers/scsi/scsi_lib.c
> @@ -1247,6 +1247,8 @@ int scsi_prep_fn(struct request_queue *q
>        struct scsi_device *sdev = q->queuedata;
>        int ret = BLKPREP_KILL;
>
> +       if (!sdev)
> +               return ret;
>        if (req->cmd_type == REQ_TYPE_BLOCK_PC)
>                ret = scsi_setup_blk_pc_cmnd(sdev, req);
>        return scsi_prep_return(q, req, ret);
> Index: usb-3.0/drivers/scsi/scsi_sysfs.c
> ===================================================================
> --- usb-3.0.orig/drivers/scsi/scsi_sysfs.c
> +++ usb-3.0/drivers/scsi/scsi_sysfs.c
> @@ -322,6 +322,8 @@ static void scsi_device_dev_release_user
>                kfree(evt);
>        }
>
> +       /* Freeing the queue signals to block that we're done */
> +       scsi_free_queue(sdev->request_queue);
>        blk_put_queue(sdev->request_queue);
>        /* NULL queue means the device can't be used */
>        sdev->request_queue = NULL;
> @@ -936,8 +938,6 @@ void __scsi_remove_device(struct scsi_de
>        /* cause the request function to reject all I/O requests */
>        sdev->request_queue->queuedata = NULL;
>
> -       /* Freeing the queue signals to block that we're done */
> -       scsi_free_queue(sdev->request_queue);
>        put_device(dev);
>  }

This patch seems to resolve the block/scsi null-ptr de-references in
our libsas/isci environment, we have yet to try James' alternative
[1]. Do we potentially need both?

Commit 86cbfb56 moved scsi_free_queue to __scsi_remove_device() but it
seems only the "sdev->request_queue->queuedata = NULL" needed to be
moved?

The conversation appeared to be awaiting test results...

[1]: http://marc.info/?l=linux-scsi&m=131007155700831&w=2

--
Dan