linux-kernel - Re: [PATCH] fix NULL-pointer dereference on scsi_run

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Date:	Tue, 07 Aug 2012 09:30:25 +0000
From:	Bart Van Assche <bvanassche@....org>
To:	Chanho Min <chanho0207@...il.com>
CC:	Mike Christie <michaelc@...wisc.edu>,
	James Bottomley <JBottomley@...allels.com>,
	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
	Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH] fix NULL-pointer dereference on scsi_run_queue

On 08/07/12 08:53, Chanho Min wrote:
> On Tue, Aug 7, 2012 at 2:56 AM, Bart Van Assche <bvanassche@....org> wrote:
>> Indeed. How about the patch below ? Scsi devices are removed from
>> starved_list after blk_cleanup_queue() and before put_device(). That
>> guarantees that inside scsi_run_queue() get_device() under host lock
>> will succeed.
>
> Thanks, IMHO, it's harmless and the simple way to solve this issue.
> But, I think the second half of your patches are not required, extra
> referecne is might suffice?

I'm afraid that without the second half of that patch the following race
is still possible:
- sdev reference count drops to zero while scsi_run_queue() is in
  progress and while that sdev is on the starved_list of its SCSI host;
  scsi_device_dev_release_usercontext() call is scheduled but not yet
  executed.
- scsi_run_queue() takes that sdev off the local starved_list.
- scsi_run_queue() calls get_device() and that call fails since the
  sdev reference count is zero.
- scsi_device_dev_release_usercontext() gets scheduled and frees the
  sdev.
- scsi_run_queue() proceeds and calls __blk_run_queue() on a freed
  queue, which is what we were trying to avoid.

Bart.
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